Curious how you know it was easy? A colleague of mine is a landlord and it seems externally to be a huge pain. Definitely put me off of trying to do the same.
In my condo building a lot of people who have moved out (out of town, out of state) they keep renting their places out with the help of a local realtor/manager that takes a cut and is supposed to deal with stuff. They don't really, and the volunteer condo board ends up dealing with a lot of the loose ends.
Based on the how much rent is in Boston are, there is a strong , strong, strong incentive to rent out. I could make about 3x my mortgage/fees by renting my place in cambridge which I bought early part of this century (of course where would I live...).
Since we have about 50% owner occupied, and the tenants are paying a lot, so they expect a lot.
Its becoming a city for rich people and students. I can understand the resentment. I think it hurts the region.
As acomjean noted, if you can make 3x your mortgage/fees as a rentier, and the rents rise every single year, you're being easily buoyed upward by the market.