the alternative is that I don’t rent out part of my house and then there will be less housing

This is only part of it. The "housing shortage" exists not because there aren't enough homes, but that there are not enough homes on the market. Truthfully, renting out a spare bedroom is not the focus of people's ire (though through a certain lens it is still a problem, but I won't go into it here). The problem is that rent seekers are pricing people out of the housing market, which is creating higher demand for rentals, which drives up the market price, ect. It's a systemic problem, and not necessarily one of individual culpability. Another part of the problem is the commodification of homes: any action taken to address home affordability will necessarily drive down home values (they are the same thing, after all), and many people depend on the value of their home not dropping. It's a bubble with millions of people at risk of loosing their homes if it pops.

There's this convenient assumption for landlords that the rental market is full of people who simply want to be renters, or full of people who simply can't afford to purchase their own home (usually by some moral failing), when the reality is that rent seekers are creating the problem that they claim to be solving. Houses wouldn't be so expensive if there weren't so many people buying houses for the purpose of renting out.

All these cucks can blow me.

Of course, there are other reasons why people might be angry with landlords.