Probably a stupid question, but why can't these lost pilots just maintain their attitude indicator in the middle, fly straight, grit their teeth, and wait to get out of the cloud?
Sometimes there are genuine issues with instruments. As well, note that most GA airplanes operate their crucial instruments and gyros via vacuum pump and they need to be re-calibrated after a while even in regular flight.
But I believe much more frequently, inexperienced humans distrust their instruments and trust their instincts/guts/feelings/body more. It's very very unnerving for inexperienced pilot to lost their sense of sight, ALL of their body is telling them they're turning and need to correct it, and some 1972 vacuum gyro is telling them they're going straight and don't touch anything.
Other times, things just accumulate, little drifts and changes and influences. Let's say that you're in cloud and trust your instruments, but after a while for whatever reason your VSI (Vertical Speed Indicator) is showing that you're descending 500ft/minute. What control inputs do you provide to correct that, without any visual references? There are right and wrong answers and integrating the basic six instruments, building correct mental picture of your position and angle and direction from these very limited and separate instruments, without visual reference, for long periods of time, is more tricky for untrained personnel than may be immediately obvious :-/
I was practicing increasingly denser fog/low visibility ILS landings of the F-16 in DCS, on my TrackIR'd 32"x3 triple screen sim rig (with transducers for buffeting sensations etc).
With triples (and much more so with VR, I'll transition to a Crystal "soon") you do develop a semi/meta(?)-"physical" connection with the sim after a while.
At a point visibility got so reduced I would completely experience vertigo. Even though I told myself it's only a sim, just look at the instruments (and the F-16 has so many, including HUD with the ILS glidescope), my head would spin out and I'd completely bork the landing.
Only after several more hours was I able to do it (with visibility so bad that the runway would only pop out when right on top of it metres from the ground). It would take a lot more to get comfortable with it, but by that point I had enough and - I still have nightmares about it. :)
Can completely understand how cloud based vertigo in a real plane with your life on the line has killed and will continue to kill so many. Mad respect to the real pilots here who have mastered this.
Most sim pilots can fly on instruments without a problem, because it's precisely the conflicting information between the instruments and your vestibular system that makes it easy to get disoriented. Flying a sim, you'll never have your ear telling you that you're leaning on your side even though you're flying straight and level, or vice versa, so you'll never be tempted to side with your vestibular system.
In a real plane, just turning a tight turn and suddenly looking down on the floor because you dropped something can induce conflicting vestibular information that's quite disconcerting even in day VFR.
Moreover, anybody who can fly a plane should intellectually know how to read an attitude indicator. But it takes a lot of training to be able to trust that indicator, when you have powerful sensory evidence that makes you instinctively feel the aircraft is doing something that it's not.
See also: https://en.wikipedia.org/wiki/Sensory_illusions_in_aviation and note the long list of accidents at the bottom.
[Coriolis illusion] can produce an overpowering sensation that the aircraft is rolling, pitching, and yawing all at the same time, which can be compared with the sensation of rolling down a hillside.
Oh, wow, ok, that sounds strong indeed... I'm starting to understand it would be super hard, while vividly feeling like I'm rolling down a hillside, to try and tell myself: "oh, I just need to focus on the instruments, and trust them..."
It's hard to imagine just how strong the feelings are from our current position of at home and typing on phone or computer, but it really genuinely is your entire body completely convinced one thing is happening, while in fact something else is happening.
(Note this is part of the reason why even with full visibility so many people get nauseous - subconsciously what your body thinks is happening and what your eyes tell you is aftually happening, disagree - sometimes strongly)
Once blind, your first prio is to keep the speed and vertical speed indicator constant. The problem is, without artificial horizont you cant really tell your bank angle, and your feelings just straight up lie to you. So you think you going straight, but suddenly your speed starts increasing. So you think "I am prolly nose diving a bit", and pull to compensate, only for speed to not decrease as much as youd like (or not at all), and suddenly you feel you are pulling 3g, and then you notice your altitude is dropping like crazy.
Thats how you get into diving spiral, and worst thing is you still have no idea if you are spiraling left or right. Keep it up for few more seconds and you can easily exceed safe speed, which given the g-forces is much lower than actual maximum speed in clean air. And a second or two later your first wing decides to go different direction than the rest of the plane.
I tested this with my instructor once, where we flew into a cloud just slightly above its bottom edge, with the goal of "try to fly trough it", only to find myself spinning out of the bottom with exactly this situation. We were prepared with brakes, lots of space beneath us and cleared general area of other planes, and checked beforehand theres no themral beneath to suck us higher, but that memory clearly showed how unintuitive and dangerous flying trough clouds often is.
Mind you that level flight through clouds only works if there are no buildings or mountains ahead, and keeping a straight flight path is even harder.