At the risk of being in the "moron" class.....
I've heard that many bass players are nothing more than frustrated guitar players who for one reason or another decided bass would be easier than learning to play 2 more strings and play chords and leads and things.....
I have known a few who did fit that category.
Then there are the ones who know less than nothing and somehow or another managed to learn the 3 note pattern to a RR or simple country song and that's how they play everything. By memorizing the notes and their positions. I played in a band with a guy like that too.... he owned the PA and the truck.... and he told me that he bought them so he would be "needed" in a band. He actually had a pretty good memory and I was able to teach him some pretty complicated tunes in this manner. In the end.... it worked.
Then you have the guys who actually know how to play... I mean really play the bass.
I have owned a bass or two in my time....and playing it, while on a basic level is fairly easy... to get it to sound good without buzzing the frets and sloppiness..... does take some effort.