Fidelity creep is also a thing. Unless you're going for a deliberately retro or stylized look, you need high detail assets and lots of them on screen at once. Otherwise you'll never hear the end of "gaem bad cuz PS2 grafix lol."

Environment textures are huge. Main character textures are huge. And you're not loading just one file, you're loading multiple files per model. The diffuse map, the specular map, the reflection map, the normal map, the subsurface scattering map for any organic models. And gods help you if your character model has interchangeable parts, because you'll be loading the whole set of textures for every element of those as well. These things add up very quickly.

And you still need space in the RAM for your code and physics and worldsim calculations, animations, everything going on under the hood. So I dunno, 16 doesn't sound outlandish these days.

Using a single stick is worse, you lose the dual memory capacity. They are meant to be used in pairs. And at the speed of the worst stick as well.

Computers aren't designed or meant to operated that way, so it will never be a requirement or specification.

Laptops with 12gb use an 8 and a 4 stick, losing all dual memory capacity. They are cheap, and make a worse product for it. Just use 2x8.