Video games are addicting to certain people. Era is just one of them.
There are various reasons to be addicted to era. Everyone might have their own reasons why it's difficult to quit.

Friends in era
As you play era long enough, you meet new friends. You enjoy talking with them, basing, etc. It's a stranger on the other side, that you enjoy hanging out with.

Updates in era
If era doesn't receive updates, it's easy enough to quit. Despite era may receive small updates, some players don't want to miss out on limited edition items. Most notably the unique snowball launcher which is one of it's own kind. The other being the gas launcher. Snowball launcher as of right now seems to be the only grenade launcher type weapon in era, and it was available only during Christmas. This is an example of making sure you don't miss out on limited edition weapons.

Unique combat system
There aren't too many games out there that have a similar combat system with era. Era allows you to dodge bullets, but also only shoot in 4 directions. Most of the time, top view shooter games allows you to shoot 360 degrees with bullets literally going at bullet speed, thus making bullet dodging impossible. If there was a game that is pretty much like Era, then you have options.

These aren't all the reasons why people can't quit era, but these are just a few reasons why some can't quit era.