I think the opposite is more irrational. I want to be able to sell my work, and I want to own my work for myself too, otherwise I have no chance of competing against the big corporates. GPL licenses don't make any sense if the authors don't control the copyright, so open source would die.

Yes, it's arbitrary - just like practically everything else. Having to drive my car on a road and not kill anyone is arbitrary, but leads to good outcomes.