One of the problems is the patent office may grant an extremely narrow patent, but the courts just might interpret it broadly. However, the shell company suing you has no assets to recover over a bogus suit. So, taking them to court has zero upside, costs money, and has a tiny chance of costing a lot of money. Thus troll.

Personally I find the whole idea of IP to be a little too strict. It is possible for me to come up with an idea completely independently and still get sued because someone got the same idea and decided to make it his IP.

Some big tech companies have the same strategy as patent trolls, and for some reason this is a much more widely accepted practice.