This is only a guess…There could be many many reasons.
1 – Recall many mortgages have been packaged, sold, resold, etc… There could be restrictions that the underlying asset needs to be owner occupied.
2 – Becoming a landlord also means incurring liability. In some cases substantial liability.
3 – Sometimes, being a landlord can be a pain in the ass.
There are plenty more reasons I am sure. There may be for all I know plenty of lender owned properties that are being rented. However I honestly do not think that is the case.