Because they can move fast enough to not be in the way for people behind them, since they are among the fastest vehicles on our roads. Bikes are considerably slower, which makes it more of a nuisance for those they’re sharing the road with if they can’t easily be passed.
Bike lanes are a good thing, and being courteous is a good thing- that goes both for passing when safe and being respectful of bike riders when you’re driving a car, and also for allowing cars to pass where possible when you’re moving significantly slower than the average traffic speed on a bicycle. It doesn’t have to be adversarial.