Because clubs are diverse. A club with no county players will have a very different agenda to the one with five guys on the senior panel and seven more on the minor and u21. Dual clubs will have different agendas again. Clubs also want to see the county team do well, and they are mentally trapped by the current status quo.
Above all, there is no easy solution to this, and there is no solution that won't have a knock on effect somewhere else. It's easy to have big ideas when (1) you have a head guy who's full time job is just to look after and advocate for your interests, and (b) you don't have to worry about other points of view. The GPA looks for stuff and they let the GAA worry about the guy on the other side who's going to have to take the hit or write the cheque. Your average club player might also be a seller of lotto tickets, a trainer of under-12s and a brother to a county man - he's by definition going to look at matters through a wider lens.
Dessie Farrell can spend a week researching a plan, studying possible ways to implement and canvassing opinions. A club player doesn't have a figurehead that can spend that kind of time on his behalf, much less one that's paid by the GAA itself, the organisation that's supposed to cater for him.