Did the landlords with troubles here not check credit reports?

There are many posts here complaining about bad tenants and some asking for opinions about certain applicants. I wonder if
1. Those bad tenants made troubles even with good credit score or
2. They don't have good credits or
3. The landlord didn't check for the credits.

I personally wouldn't rent to people with bad credits for I have a perfect 815 credit score myself. :) (Can anyone have 900 full score?) And I personally wouldn't rent to certain kinds of people. My houses are far below 1:100 but good night sleeps are much more important for me (this is why I don't invest in stocks, commodity, etc.). I guess my principles wouldn't work for professional landlords. But if you only rent out houses to keep your extra cash above inflation, it works fine.