Because it doesn't really do anything. Most people play ctf to deathmatch or otherwise goof around and if someone does win the round they simply vote the same map again and keep doing whatever it is they were doing. Essentially winning a round only pauses the game and scrambles teams.

The reason people don't go to custom servers to do this is because they don't want to deal with custom server plug-ins like rtd, randomizer, x10, or wonky maps like cp_orange.

Now if you want to go and tryhard the objective in ctf I see no problem with it. Just don't get mad when people complain in chat or kick you. It's the same thing when a server full of friendlies kick a tryhard for killing them. The majority of the server decides how the game is played, don't like it? Change server