It is sort of publicly available, but in an inconvenient way and you have to be dedicated to get an aggregate picture. For an individual building you can look at a few different things. I look at the address to which the tax bills are sent. This is usually a good way to estimate how many parcels are owned by the same entity. For a specific building, I search recorders records for the name of the entity I think owns it. This will turn up all the recorded deeds with the same entity name. Problem is many buildings are owned by single-purpose entities so then you have to also look in county fictitious business name permits and state business licenses.