If the RNG is bad, you'll get more benefit from adding non-random bits than you would from additional badly RNG'd bits.
The probability of future collisions also rises the more IDs you generate. If you incorporate non-random bits, you can alleviate that:
- timestamps make the collision probability not grow over time as you accumulate more existing UUIDs that could collide
- known-distinct machine IDs make the collision probability not grow as you add more machines
There is a long history of broken entropy sources showing up in real systems. No matter how hard people try to prevent this it keeps happening. Consequently, a requirement for high-quality entropy sources is correctly viewed as an unnecessary and avoidable foot-gun in high-reliability software systems.