Who writes all these viruses ?

I mean, I've experimented with assembler when I was a teenager and I may have developed some kind of program which could replicate itself.. but I highly doubt today's viruses are written by teenagers...

Who and why do people write viruses ? Is this a thing at all or are all the viruses written by the Antivirus makers themselves ?

More 'threats' is good news for the A/V makers so why not have a separate department which develops them ?

I wouldn't be surprised at all, given that much crazier things are happening in this world..

Can anyone confirm or disprove this ?