It's a law of large numbers thing. Americans romanticized mom&pop landlords vs big greedy landlords, but.. it's a bad business to be a smalltime landlord. It's like putting all your money in one stock.

If, say, 5% of the population is crazy, and make for bad tenants.. then owning 10-20+ units puts you in a position of always having 90%+ of your revenue coming in.

If you have 1 unit then most of the time you are OK, but every once in a while you may lose 100% of your revenue for 3-12 months, while you have to keep spending on mortgage/tax/utilities, plus lawyers, repairs, etc.