These players used to have outragously high wages in the past and today they could not be economically sustainable for common clubs, since the single wage cost plus the other 'normal' wages would cause the team to go bankrupt.
A farm team does not have enough skilled players to stay in a high division.
Farm teams may have low enough wages (aside from the farmed player) to remain viable; or they may have a huge bankroll from money hording or selling off their talent that is slowly bled by the wage bill.
Farming is the practice of growing one player or more, to the supposed detriment of the rest of the team. Farm teams train a player, often a National team player to tremendous heights. They cope with the immense wages by using cheap players everywhere else, and often only play one game a week. Some managers log in every 6 weeks only to avoid losing team.
Opinion about farming varies. Many users see farming as another approach to a game played for diverse reasons. Others decry it as unrealistic and a ploy to get national team members beyond the level non farming teams could provide.