Why is keysize almost always expressed in bits? I've never actually seen a time when somebody has expressed RSA keysize in bytes, and I haven't seen keysize expressed in bytes in other encryption algorithms either (can't be certain that that's always the case). Is there a reason why keysize is almost always expressed in bits instead of bytes (in encryption algorithms in general, not just RSA)?
P.S. if possible possible, please link any documentation to support your answer.
It’s traditional.

(Don’t underestimate the power of Tradition. The main reason we use bits is that this is how things have been done for a lot of time. If only to avoid confusion, it is a reasonable idea to stick to traditions. You don’t need a compelling reason to do things in the usual way, but you’d need one to do things differently.)

Also, bytes have not always been octets. The PDP-11 used 9-bit bytes. Even the C standard defines bytes to have at least eight bits, but possibly more.