The real problem is that we don't have something analogous for microphones. The best we could do is some kind of humming device that you could turn on near the sensor that would drown out any noises it might otherwise pick up (and also be unobtrusive).
It is definitely not low cost.
In the hardware business, we count pennies. We count fractions of pennies with the costs of various components.
A shutter means at least one extra mechanical part, and a more complicated housing and increased assembly time at the factory.
You might look at a piece of plastic and think it is only a few pennies of plastic. But by the time you get it integrated into the product, and shipped out to consumers, it can add up to a dollar of cost. That really eats into the profits of low-margin hardware.
Only if enough consumers care about hardware shutters does it make business sense to include it. If most don't, and it is not a part of their buying decision making, then you're just throwing away money. Especially if it leads to increased post-sales tech support costs: ("My camera doesn't work! Is the shutter closed? Oh it is!").
Note that this is not an argument not to have hardware shutters. They are a good thing. But they come at a significant (to the final price) cost.
In any case, I don't think that's relevant to my point: I specifically said "important for laptops to permit a physical slider to block the integrated camera". That phrasing was deliberate: you don't want to trust the OEM's slider in the first place, just be able to add you own.
So, am I addressing a strawman? What OEM impedes you from adding a slider? Well, actually, Apple says it's not recommended to use a webcam cover because of how they designed it. That is poor design and security practice.