Note the '2014' in the title; this is one of those regular sports games franchises, and it has been around since 1992 (For the first 10 years or so, it had the name 'Championship Manager', but that was changed due to legal hassle). Every year, these guys upgrade their engine, upgrade the player database, pile on some improvements and tweak stuff, and put out a new game. Some fans buy the game every year, some every two, or every three years.
It might be a niche product, but Football Manager owns that niche; it has a recognisable brand name and product (one that is well-recognised in the real-life sport itself to the extent that one Premiership Football Team - Everton - even bought early access to the player database), a large and fanatical fanbase (the company has fans in almost every football team in the world scouting players for it), and constant (if not growing) revenue stream.
Gamers aren't stupid; trying to nickel-and-dime them with those hated microtransactions is the sort of thing that would easily lead Sports Interactive down the road to an EA Dungeon Keeper-style PR nightmare. SI are already onto a very good thing; the sort of quick-buck Zynga/King style cynical disruption you're suggesting would be a total disaster.
Rather than "hooking" its players, you could read it as, this game has a devoted cult/niche following. One of those things certain people love & cannot do without, yet has narrow appeal.
Kind of like snake antivenin.