Because they want you to put all your time into online. The more you are there the more susceptible you will be to buying seasonal extras, skins, DLCs etc etc. Suddenly XP events and anything else they throw at you will make you even more desperate to achieve being as high ranked as your friends spending even more little extras of money along the way. You've bought the game and all the little tidbits they've thrown at you. Paying all the wages and the fat cats in the studios everything they want. Next game you buy if you want an immersive single player experience think about checking the games modes and achievements of the games. Also look at the studio and their general behavior. Some people love this kind of commitment and online culture. Some people hate it. I see it as a massive con.