Ghasts spawn very close to the player in the nether, and spawn far too often. When in open areas, a ghast will often spawn immediately after I kill another one. Multiple ghasts will sometimes spawn at once. From what I can tell, these ghasts tend to spawn when I'm loading in chunks, but I can't confirm this is 100% accurate. Render distance is set to 10, simulation distance is set to 4 (I'm playing on a realm). I've experienced this while alone on the realm or alone on the nether. I don't know if this happens with multiple players in the nether.

I've attached a few example clips, the 3rd one shows a spawn too close to the player. The other 3 are some examples of how frequently ghasts spawn. Clips 1, 2, and 4 happened within about an hour. In these clips you can also see ghasts sometimes despawning very close to the player, but this might just be the result of the low simulation distance on realms.