"Hotel/booking devs, why is it impossible to filter out hostels?"
Other than setting a high minimum price, there seems to be no reliable way to ensure that search results are not polluted with artificially low prices caused by hostels that offer cheap shared rooms as well as expensive private rooms.
Booking.com even has a separate option "only show me private rooms", which it ignores.
The problem is that in some countries where costs are generally lower, the difference in price between a hostel room and a low priced private room in a less popular area is small enough that simply raising the minimum search price will not help. You'll filter out some good options and still have some hostels in the results.
Selecting property types, intentionally excluding hostels, does not work either.
The key problem is that many hostels have some private room options in addition to their shared offerings. So the hostel will appear in your hotel results with its cheapest shared room price.
After years of traveling and fighting with search sites, I have come to the conclusion that there must be a good reason that the major sites - Hotels.com, Agoda.com, Booking.com, and even Google's hotel search cannot or will not allow us to filter out hostels. What is that reason? They must know the problem exists.
I generally avoid the low-end properties but I’ve seen the problem you describe. I draw the line at shared bathrooms.