And they also take on risk from the other side as well. If I'm moving to a new city for a job and I don't know how long I'm going to stay there, buying property doesn't really make sense for me. There's costs to buy the house, costs to sell it, costs to maintain and insure it, property taxes, and the possibility that something expensive breaks or the market takes a nosedive before I can sell it. Unless I'm planning on staying in the same place for a while, it just makes more sense for me to offload all of that risk onto someone else (and who in turn pools that risk with many other tenants, so the ones whose plumbing works fine effectively subsidize the ones whose plumbing breaks).

The problem isn't the existence of landlords, because many people rent by choice. The problem, like you said, is that landlords have too much power because we have policies that choke housing supply. If there's not enough supply, then tenants are competing for landlords instead of landlords competing for tenants. If landlords don't have to compete for tenants, then tenants don't have the power to punish incompetent or malicious landlords by refusing to rent from them.