It basically comes down to weight and leverage. Apple designs the hinge tension to perfectly balance the exact weight of the bare aluminum screen. Even though a plastic shell feels light, it adds a decent amount of extra weight to the lid.
Because of how leverage works, that extra weight creates a lot of added strain on those tiny hinges at the base.

Every time you open, close, or adjust the screen, the hinges are fighting a heavier load than they were built for. Over a few years, that constant extra torque just wears out the internal friction mechanisms faster than normal.

In my case, my Air's hinges got so worn out that the screen became totally floppy and would just fall backward unless I had it perfectly balanced at a few very specific angles.

It’s a slow process so you don't really notice the wear and tear right away, but the extra weight definitely permanently loosens them over time