Backwards compatibility, it’s never been valid to send a body with a GET, and software does enforce that (to varying levels)

It’s easier to create a new verb with the wanted semantics, than to change an existing one.

you can still send a body with a get, it’s just up to the server to accept and parse it.

I suppose you could use a proxy like nginx or apache and rewrite the body to params, but that’s not really what GETs are for.