As a landlord I've had to put up rents on both my BTLs recently, and I agree that they're extremely high now. The monthly rent is higher than most people's monthly income. (Although having said that they are in Prime London locations).
Factors like stamp duty, taxes and increasingly hostile legislation against landlords do play a part in that, but the main reason for it is the high inflation, and in turn, the extremely high interest rates we're seeing at the moment. The monthly mortgage payments on them are now almost double what they were a couple of years ago, and it's a similar story with servicing and maintenance costs. So if I didn't increase rents I'd end up operating at a loss, which wouldn't be sustainable for long and would force me to sell at basically bargain prices (probably even less than the amount I've spent on them in the first place).
I feel like landlords often get demonised for just being "greedy" when increasing rents, when in reality there isn't usually much of a choice in the matter. It's either that, or not providing rental accommodation at all. The anti-landlord rules and taxes that keep coming in don't make the situation any better for tenant either; they just make it more expensive and more competitive to get a rental property. I get that what people really want is for houses to be more affordable to buy, but penalising landlords just for being landlords doesn't really help that. Ultimately the problem is down to the simple fact that our population is high, and properties are scarce. There's no getting round that other than by building more properties.