Because football is full of idiots.
In all seriousness, it's financial pressure at the root if this I think.
The cost of relegation, the reward of promotion. It leads to short-term thinking and decisions.
Developing and more importantly sticking to a clear and coherent strategy seems very difficult for most clubs. A bad patch of form and the pressure builds insanely quickly.
When you get an accumulation of short-termist decisions, you just end up in a mess. Everton are a fantastic example at the moment.
No clear recruitment strategy, no clear managerial strategy, no clear style of play, bad bad football.
There are concepts that seems so simple, yet are forgotten.
Brighton have a style play, they recruit players that fit that style, that makes those players look really good, they sell them for lots of money because they look good. When the manager leaves, they recruit another manager that has a similar style of play, hence you don't need to completely overhaul the squad.
It's not really that complicated, yet so many clubs get it so so wrong.