I'm not sure why some places require 5x rent in income it is wildly excessive. My personal theory (based on hunches only I have found no evidence for this) is they want to only attract tenants who are less likely to leave after a year or two of 10% rent increases. They may also be trying to push gentrification by making sure only well off people enter a neighborhood, fuelling the need for more "luxury" apartments to be built and perpetuating their industry. Idk, they're pretty tight lipped about the reasoning if someone has more insight into their motives I would love to know

The idea is they'd prefer to be picky because there's practically infinite demand and artificially low supply. They can vet all but the most guaranteed to pay out, mitigate risk, and make more money.

Maintenance & overhead costs could mean that it’s not profitable, or barely so, which means they could be employing some strategy to cause a sale. That would give them massive funds to find more profitable & less risky endeavors.