Because it means losing control over their brand. Who knows what some random youtuber will do, who they are, etc. It's just not worth all the legal headache.
They do it anyway. And it saves you the cost of the cat and mouse game. You cannot win this unless you create a dictature. There is Mickey porn and fascist Pepa pig anyway. So why not collect the benefit of having real fans making great content instead attacking them.
E.g: nintendo killed an incredible fan made metroid game. It seems so short sighted.