Thank you! Nicely put. The problem isn't people like your aunt, its massive shareholder-controlled investemet machines that own thousands or even millions of homes. Your aunt probably knows eafh renter by name - there can exist a personal relationship. There's two things limiting your aunt becoming a money-hungry antisocial ghoul:

1. raising the rent is a relatively large amount of work for relatively small of a reward. If she raises rent she has to write these 4-5 renters a letter explaining why she has to increase it. Those renters might disagree, have objections, ask for reasons and proofs (like the central heating bill or maintanance costs etc). If she raises the rent by lets say 2% it's 2% of not that much money (with her single digit number if houses).
2. she is raising the rent on people she knows. She is taking money away from people she even may like - have a personal relationship with.

So increasing rent is a lot of hassle and her renters might like her less after that - which might be a factor.

Now lets think of the hugr real estate company. They have thousands of renters and maybe hundreds of employees. They have lawyers employed. If they raise rent by 2% they have to send thousands of letters. But these letters are sent by people whose job it is to do so. Tyey can calculate in advance that from their renters X% will just accept the nrew rent, Y% will require some manouvering, Z% might move out and so on. They can estimate the cost of raising rent pretty well based on experience and compare to the profits they make. And with thousands of apartmants 2% is a lot of profit. The employees have no relationship to the thousands of renters. Renters are just numbers anyway. Everything is much more efficient. Also: Shareholders. They demand profits and dont't care how. They care even less aboht the renters. They demand more profit and will just say "make it happen". If thr ceo doesn't raise profits - with whatever means necessary - the shareholders will replace the ceo.

The soltion IMHO would be some progressive tax That makes it basically unprofitable to have more than 10 apartments. And to prevent legal entities owning other legal entities owning apartments in order to circumvent this. If there exists (and can reasonable exist) a personal relationship between landlord and renter everything is alright in my opinion. People usually are not animals to eaxh other if they know eaxh other personal.