Ghasts really aren't that hard to find though. All you have to do is find a medium sized area of the nether big enough to spawn a ghast, like near a fortress, and travel around in that area only looking for ghasts. Maybe go back and forth in a specific path or something. You aren't going to get several ghasts at one time like you would with a spawner, but doing this makes finding ghasts easier (it made it easier for me at least, I found a lot more doing this than I would have if I was just regularly going around the nether).
Also, I'm sure that ghast spawners aren't a thing for a reason. Personally, I think that would make survival a little bit too OP and I don't think it needs to be any more OP. Especially with all of those bigger parties, getting ghast spawners would only make them richer. This is a different conversation for a different day, but I think survival needs nerfed a little bit.