Because in many cases, those administrators don't even realize they're breaking the network. They just run a botched config that they inherited from their predecessors or something like that.
You might say that they're clueless and shouldn't be allowed near networking equipment (and you might even be right) but it's not going to change a thing. For the foreseeable future, working around broken environments is all we can do.
Home routers is another matter, home users don't make conscious decision about it, but in my experience UDP works just fine (because a lot of games use it) and it should be good enough for any protocol.