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've called them bikes, quads, four wheelers, or atvs...just whatever comes in my head at the time. Don't have a reason for calling them one name over the other...
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