The root cause

If the idea described in the patent is very general then you have a problem.

i.e. if the patent is "how to cure rubber" it's bad

But if the idea described in the patent is specific, it can be worthwhile and doesn't prevent other people from achieving the same result through different means

i.e. if the patent is "how to cure rubber by following this process for calculating how much heat you apply to the rubber" it can be good.

This doesn't prevent someone else from inventing another way to cure rubber, they just can't use that specific approach to the problem. If they come up with a different way of calculating how much heat to apply, that can be worth a second patent.