It's simply an addiction/habit, I was the same in previous FIFA's and still go through stints on the newer ones where I play more/longer than I really should be doing.
Funnily enough, the game stopped being fun for me when they introduced the rewards, it's sent prices to a level that are similar to when coin buying was prominent and driven 95% of players to be as sweaty as possible, I mean even when something half decent is on offer, in weekly objectives for online single matches, you have everyone playing like an absolute nob jockey.
It isn't rewarding at all and I've had some decent red picks and pulled some fantastic untradeables. I couldn't even tell you why I still play it, other than "I like football" when the game plays nothing like an actual football match.