games are very addictive and as ya'll said, very much designed to be that way. The achievements/trophies (cheevos) that Keith mentioned aren't even that bad. You have to actually play through the game and try different things to get some of them. Most games give them to you for doing nothing, beating the first stage or finding a clue. They don't make anyone feel that rewarded anymore.
There's legislation going to the system now to ban loot boxes from games because they are literally just little sections of gambling that kids have been taking part in for 4-5 years now. Games like Overwatch, Destiny and apparently the new NBA and NFL games have unlockables that affect the game in significant ways that you can pay real Earth money for, to get a CHANCE to unlock. They have people straight playing a slot machine.
Some Asshole at a pretentious game blog wrote this article defending loot boxes recently. Saying that without them, the billion dollar companies who utilize them would be too poor to pay devs living wages and hold tournaments. But only if you ignore their aforementioned billions and the hundreds of millions they pay CEOs for doing nothing but hiring people to come up with things like loot boxes.