There are a good many guitarists that have memorized the licks from the tab sites, but just plain suck as musicians. Not all, but many.
At the extreme low end are the ones that play the wrong the chord changes, are rhythmically challenged, don't know the difference between a minor and a major, and strum an open D chord with E in the bottom, always strunm a six string chord, but...they have really good equipment and can play really loud and grunt while they solo, so they fancy themselves great musicians.
And of course, since they are the guitarist, they are naturally gifted as musical leaders and multi-intrumentalist.
I don't mind other musicians copping the bass line. I have seen some jazz guitarists walk the bass while they solo chords. And organist Joey De Francesco has the baddest azz bass lines this side of Ray Brown.
But then again, many guitarists are not really good musicians and are oblivious to that shortcoming. So, of course, they think they can play bass.