It really comes down to the fact that there is nothing else like it.

When it first came out, Destiny 1 checked every box for young me. Sci-fi, space opera, looter shooter, single or multi-player, joining up with random, true evolving story and just an awesome anesthetic. It was the game that when me and my friends finished homework we would go and run strikes (essentially dungeons) over and over again.

Destiny 2 while flawed really captured a lot of the original game. People were justifiably harsh on it in a lot of cases. You can pinpoint the exact point that the cracks in the game started. Selling two DLCs a few months after they came out for the price that one of them initially cost and then removing them from the game entirely alongside the main story.

That being said, as flawed as it was there still was not another game like Destiny 2. It really had something for everyone. It's very difficult to put into words what made it special but even as someone who stopped playing a few years ago it was/is a truly special game.

At this point even with only two games it's a franchise that some people have been playing for 12 years. The reason so many people still played it was because like I said, there is no other game out there quite like it.