I believe the main reason is a lack of research and planning,then financial.To many people get sucked in to thinking repairs and upgrades are a snap that any one can do with a 12mm socket and phillips head screwdriver.
How often have we read about bolt straight in transplants and up grades to brakes,suspension etc.etc.the list goes on and on and it's in the main a load of crap,requiring far more than remove and refit in reality.
I know most people mean well when expressing thier views on this forum but sometimes the devil is in the detail,and this is not fully explained at times and the un wary get sucked in cause they have not researched thier intentions fully,including tools and skills required to perform these tasks and in time loose interest.