From my experiences, it's because they gain some sort of power and superiority by putting others down and of course, society rewards the assholes and jerks, while lying and virtue signaling to the victims that the bad guys will ultimately lose. I don't want to dwell on the past, but I'll just say that the people who bullied, were mean, tease me in early HS, most of them lead fairly successful lives, while maybe one or two ended up with a tough and shitty adulthood. Personally, I wouldn't consider myself successful in the adult world (having a decent paying job, living independently from parents and having my own apartment/place), but that's a different story.
Not all bullies end up with karma biting them in the ass, despite what idealistic bullshit people spew. Here is one such example:
This should make anyone rather angry. While I wasn't bullied like that person, I had my fair share of teasing and harassment by assholes during my teenage years. I don't hate them nor resent them, but it wouldn't make me sad if they got their comeuppance either.