It doesn’t have anything to do with indexing more than one thing.

The issue with truly random IDs is that inserts are scattered throughout the B-tree index, which cause page splits, fragmentation, and poor cache locality. Using a timestamp prefix makes new IDs sequential, so inserts tend to land near the end of the index, which avoid all the B-tree trashing and improve write performance.

They already do. UUIDv7 neatly solves a common problem: a random, unguessable, public id that works well with databaes. UUIDv4 is infamous for this.