Maybe because it became part of the jargon and served a useful purpose --- if you mention creat, it's certain you're talking about the system call and not creation in general.
Personally, I like names which are close enough to an existing concept to be evocative of the meaning, but also distinct enough that, just like "byte" vs "bite" and "nybble" vs "nibble", a (non-stupid) search engine can give relevant results.
If "execute" was really called "exec" in the source, then none of the syscall names I saw are longer than "creat".
My understanding is that the linker didn't really limit the identifier length, it's just that only 6 characters were used to match them. So, shortening names create->create, execute->exec might have come afterward, as a clean up round to make it less surprising.
Looks like 40 to me... 64 would be 100.