Because managers are stupid and just blindly copy trends.
You get teams trying to play out from the back with clogger centre halves and club footed goalkeepers totally ill-equipped for it, or managers saying "hey, three at the back, all the cool kids are doing that, I will" without ever considering how the system actually works.
Managers are almost exclusively former professional footballers, there isn't a weight of intellect there a lot of the time.
Think about how many times a Wolves manager has done something that all of us, the laymen, committed laymen but laymen all the same, can see is obviously not going to work before a ball has been kicked.