Destiny is a loot shooter (or "looty shooty" if you're in polite company) which has rare drops as part of it's core gameplay. Certain rare guns can only come from high-level activities, and some weapons have randomised rolls that encourage people to re-play those activities to get gear that is marginally better. A lot of the best parts of the gear drop design are lifted from peak WoW. But that alone isn't what makes destiny addictive.

Destiny 1 had "gun leveling" where you would need to get kills on certain rare weapons to unlock the rare features that made them useful, along with individual numbered levels for each gear slot, and that's before you get into the quests and bounties. There was always a reason to hop on, because you always had some new weapon that you could power up to see what it did. Tiny bars filling up.

When Destiny 2 shipped, they dramatically improved the loot system, simplifying the ways people would get drops and making them more consistent overall. But they also tried to remove a lot of the other "grindy" aspects of the game like gun leveling, but the result was the game didn't feel like you had compelling reasons to log in. Fewer bars = fewer hooks.

It's only now that the game has had several significant expansions and added a host of new quests (many of which simply require using certain types of gear in certain activities) that the game is approaching those same levels of addictive.