I dont think It's hard to understand at all. Great musicians are often relegated to cover bands because they spend all their time working on their chops and becoming a good player. Writers spend their time becoming better writers.

If you work on both you can become good at both, but writing doesnt happen automatically u have to learn how.

Playing covers involves no creativity, just chops, unless of course, you do something different with the cover then you enter into the creative zone. ie different arrangements, feels. But that involves the other band members too.

How come some guitar players can play every Eddie Van Halen solo or every Jim Hendrix solo there is
and play it great, but when it comes to soloing on something original they fall way off the mark?

Cause they learned how to copy and didnt learn how to write/compose a solo. They dont have Jimi's thought process or creative process, they just have his riffs down.