Back in highschool when I had a practice after school people would see me carrying around a big gig bag and wonder whats inside it. When I told them its a bass they said the isnt bass easyer then guitar line. And I always replyed with Its easyer to become a famous guitarist then a famous bassist. Some that played guitar asked to play it. I let them and like everyone else said they play guitar riffs on it. Then I played some slap licks that I came up with and they never said bass was easyer then guitar again.
Good tactic.
And on another point, sure - some people play lots of instruments at equal ability. But most people don't play lots of instruments equally well.
In general, the skills of the typical guitar player don't transfer so immediately to bass (and vice versa).
If you can play both equally well, that's very cool. Most people can't (and some who think they can, can't, as well).
/very opinionated about such issues