A lot of bits of security rely on some level of non-determinism. Things like TCP initial sequence number generation, where every TCP connection sequence starts with a random number. There have been numerous attacks where the RNG wasn't good enough, so an attacker could determine the TCP sequencing and perform various malicious activities. Additionally, things like in-kernel VPNs, like IPSEC and WireGuard, also need RNGs for their own internal functions. Calling out to userspace for that would be painful and could potentially break in a lot of unexpected ways.