It always hurts to fix known issues - if you work in engineering, design and development you’ll know what I mean - nothing is really a quick change, but an expensive process - sometimes months of work depending on the issue. When you have to meet certain internal deadlines, you have to make tough decisions.
Sometimes people make wrong decisions - sometimes people gamble on the issue not being reported too often for example (eg the iPhone 6 bend gate).
A lot of times, the problems were already fixed in subsequent revisions.
For example, the bending problem was fixed in the iPhone 6S.
So, it's not that Apple doesn't know how to fix the problem.