Why is call of duty so addictive?

because of all the **** you can get by levelling up, it's a basic concept in use on most games, look at pokemon red, you always wanna have fully evolved pokemon, on cod you wanna get all the guns and attachments, on football manager you want to win all competitions. if you've ever played fm you will know that once you win the cl and prem you instantly can't be arsed anymore