Terminals are, generally, expected to behave like terminals, which brings along a host of legacy compatibility things about how keyboards used to work over the past 40 years. Not saying that’s an excuse, or that they shouldn’t try to, but balancing “sane modern defaults” vs “sane-ish defaults from the 70s that everyone’s used to now” is hard.

EDIT: To elaborate a little… Terminals pre-date arrow keys, which is partly why there are lots of default “movement” keybindings that don’t use them (ctrl-a, ctrl-e, ctrl-b, ctrl-f, ctrl-n, ctrl-p, etc.).