How are you coming up with stuff that you can’t actually play? Are you humming things? Working on a keyboard or other instrument? Thinking up mathy or harmonic sequences or patterns in your mind?
In general when I do that kind of thing the lines are usually more literal or linear in nature. The kind of thing you would be able to think up on your own. Whereas, after “guitarifying”, usually they are not things I would necessarily have thought of.
Obviously you’re not going to like every line on a guitar. Even if you could play them all, you might not even like them. But there’s always a way to get something you like. There’s some commando application of flat picking, finger / hybrid picking, legato, tapping, harmonics, trem bar, effect pedal, etc. We can keep going. There is some commando way to do something cool to flesh out an idea.
Whereas if I could “alternate pick everything” I bet I’d write the most boring stuff… because I would never be forced to try something weird.