I agree, if what you want is a standard chair or table. But its common that I will want some piece of furniture that exactly fits some space or is otherwise unusual. For example, shelves that exactly fit my rooms.
It does help that I have somewhat low standards for appearance, and am quite content building things out of cheap wood.
(I'm assuming that we're talking about saving money in a way that counts your own time doing something you enjoy as free.)
Also your time still has value even if you enjoy what you're doing. You could be doing other hobbies you enjoy more, or other aspects of the same hobby, or making money so that you could afford other opportunities instead. You may enjoy woodworking enough that you're willing to forego the value of that time to pursue it, but that doesn't make it any more economical.
Some examples:
* At least five sets of shelves, the biggest of which was https://www.jefftk.com/p/built-in-shelves
* Piano stand, so I could put my keyboard at just the right height on top of a wagon
* Folding monitor/TV, that is pretty sturdy and well protected when it's away: https://www.jefftk.com/p/folding-couch-monitor
I expect to keep doing this, and most of my tools aren't even halfway through their useful life.
There are some downsides, and you're right that a professional would make a lot of these things look better, but there are also serious downsides to working with a professional aside from the cost. It can be quite hard to get them to come when you want, they may not build exactly what you had in mind since communication is hard, and some people will cheat you (https://www.jefftk.com/p/brendin-lange-is-a-scammer)