Because you are not skilled enough with YOUR roller. You are dead wrong, dude, sorry. It is much more difficult to get things really smooth with rollers but it can be done as well as spray and much cleaner as well. The BRAND and specific TYPE and SIZE of the roller is very important. The SPEED at which you paint a larger area with a roller is also very important. I speak from years personal experience. You will NOT get "super smooth" results using a paint roller your first time, unless you are incredible talented and experienced as a painter. I must add though, I am a fine artist with 30 years of fine art painting experience using every kind of paint and painting application imaginable. I love Sherwin Williams oil based paints, the paint quality rivals super expensive fine art oil paints in tubes, it's THAT good (and expen$ive!). Your milage will vary GREATLY with your personal ability to paint. Go ahead and flame me but I have at least one member on this forum that will verify my painting abilities.
Good luck!