I can't really comment on anything you mentioned other than Mario Kart (since it's all I played). I still play it with friends occasionally but I've pretty much just stopped playing it online because there's just nothing to really work towards. Even if I enjoy a game I'll still tire of it if I don't have that 'carrot on a stick. This can range from level, rank, improving K/D and W/L ratios. Basically the things that give me evidence that I am making progress. Just driving around or shooting people with none of that gets old real fast for me, which is a shame since it's such an easy thing to solve by raising the level cap, increasing the amount of EXP needed to level up (so it doesn't take 2 friggin' days), a prestige or player stats screen where I can view my overall stats for certain things so I can try to increase them.
I mean, I'm still enjoying Splatoon at the moment because they're still releasing/unlocking content, I still haven't reached A+ and I'm still trying to get the stats I want on my gear (since Spyke has decided I'm not allowed to have good stats no matter how much money I throw at him). Not to mention since all my money is going on remodeling my gear, I've not had enough money to buy all the clothing available. I still have much left to do.
There's gonna come a time though where I'm just running around with nothing left to work for and all I'm really doing is pointlessly shooting things without a goal. It won't even matter if I win or lose matches at that point, so why bother?
Compare that to something like World of Warcraft on the other hand (stay with me, I'm going somewhere with this). You had 11 classes to level up to 90 (the cap when I last played) with 13 race's, than you had item level, profession levels, arena levels, ranked levels, achievement points, pet ranks, World control etc. etc.
I'm not trying to directly compare Splatoon to Warcraft, I'm not an idiot. What I am saying is that I played Warcraft for like 6/7 years without a break in subscription and 90% of my time was spent leveling up new characters and doing ranked PvP, so it shows how something as simple as adding a number to work towards can keep a player interested.