I’d say it’s because the pricing is algorithmic based off of demographic and personal data. Similar to hotels, it can be more profitable to keep some empty in order to reduce supply and price up the others.
You mean if you own a massive complex, which is the vast minority of housing - and also not what was mostly accused of being bought up by individuals or firms for speculation.
If you're talking smaller units, landlords are effectively price takers and it is pretty much never profitable to keep empty and price up everything else unless you are anticipating immediate increase in prices.
None of this is a major driver of housing prices either way.
Basic economics on how prices work? The fact that obviously everybody is a price-taker in most home markets?
Your theory is for some sort of massive coordinated action that would reap massive profits to anybody for defecting from. It's game-theoretically impossible but I need to provide the evidence? There are some landlord pricing tools that should absolutely be subject to anti-trust enforcement, but this is again: an extremely small current in modern rental/housing markets.