It's not because they wanted to, I guarantee it

It's not because they wanted to, I guarantee it. It's because it's impossible to not do it. Critically important details, down to the engine's frame timing, how replication is deeply integrated into the engine, and how the default Actor movement code works means you have to not gut just a single implementation detail, but you have to cut whole swaths of code out of use. I'm dying to be proven wrong, but the intersection of people who know multiplayer networking well, Unreal Engine 4 or 5, and have successfully implemented client-side prediction with reconciliation and rollback in said software and out of it as well, are vanishingly small. I failed to understand how one could use the existing engine's loop which is variable framerate, the existing replication system, which favors an implementation opaque RPC system over explicit payload structures, and rewrite Actor movement and engine timing subsystems without having a parallel network loop that uses none of the above. …