Won’t retroactively change things that support GET to support GET-with-body. A new verb makes it much more likely that anything that supports the verb supports the desired semantics.

(Of course, you could instead bump the HTTP version for support of GET-with-body, but given how HTTP/2 and HTTP/3 are defined in terms of HTTP/1.1, you’d need three new versions of HTTP for a change to core verb semantics. A new verb, again, is far simpler.)