Because many of us see this class of vehicle as an extension of the category of motorcycle, or motorbike.
This is likely because of riding/seating position; upright with legs straddling frame/fuel tank using handlebars to steer and hand controls for accelerator/brake.
Notice the term bike is never used for utility atvs or SxS as the seating position changes to become more car like.
I catch myself calling them bikes at times, Im not really thinking when I say it, more of a habit since I started out on Dirtbikes. I've even caught myself calling my sled a bike at times lol, everyone knows what I mean though
I use "bike" when im talking with people who are into the sport. I say 4 wheeler when with people who aren't into it. Quad just doesn't sound right and ATV just doesn't roll off the tongue. Bike is just easier to say