In Toril, creating a Floating Mountain isn't really a difficult task, but whether it's the cave-like Floating Mountain where Terry's biological parents reside, or the inverted peak-style Floating City favored by the Harua Mage, there is one major hassle... they must first find an unclaimed, beautiful mountain peak to carve out.
This used to be easy, but as more people came to Toril, things got more complicated, and as more reclusive strong individuals appeared, finding a suitable mountain peak became too hard.
Moreover, there would be a large number of Druids and Marla Devotees who would relentlessly chase after them due to their damage to nature… so now Floating Mountains are basically inherited.
The most recent example is Terry's mother's mountain, after all, no one would go to discuss environmental damage with two Ancient Dragons.
Even Marla Devotees wouldn't... but their explanation for this is that Dragons are inherently part of the natural cycle.
Humans are not.
