> What's stopping someone from just adding an additional IR light to those enclosures

Nothing.

> Couldn't you just slap an additional bright enough IR light in that makes it impossible to even see the plate clearly through cameras?

You could: but it will only work at night (and even then, I don't know if the amount of light you could concentrate in that area would be enough to blow out letters), because all of these cameras have switchable IR cutoff filters.