because they can. trying to pigeonhole cyclists as either cars or pedestrians is exactly why so many confrontations occur. its not that a cyclists prefer sidewalks as they are horrible to bike on (bumpy, uneven, inconsistent, obstacled), its that on some roads a distracted driver can mean near certain death. Its not irresponsible, its moving a 20 pound object around 100-200 lb people (if pedestrians cant be avoided) instead of holding their own against 3 ton cages.
Are there douchey cyclists? yes. But just try getting down one block of SF only in the bike lane. You’ll be stuck behind a ups truck or broken muni for days or pop a tire on a broken bottle some hobo left there or get hit in the face with a door.