I'd imagine many will answer your question with a resounding "well duh..."

Indulge me, while I attempt to point out the obvious.

The reason why this game has so much content is because it's three years old. The reason why it's three years old is they obviously have done well enough to keep enough players engaged to earn a sufficient return on their investments. We are all paying a subscription. This game is not a one time purchase. It would be in their best interest to keep enough players engaged to generate enough revenue through subscriptions.

I would speculate that one way they keep enough players engaged is they provide their audience with enough fresh content, that they happen to be interested in, to keep these types of players subscribed. Their customer base is comprised of players with a wide variety of interests, so they provide them with a wide enough variety of content.

End game content is just a fraction of the content they develop, because only a fraction of the customer base is interested in this type of content. For those players that do not have any interest in that type of content, the developers provide them with other types of content, in the hopes it will keep them engaged enough to remain subscribed. They obviously have the resources to develop content for multiple audiences, and they have apparently been successful in doing so, thus far.

They also do a great job keeping us informed as to the different types of people they have employed to keep all this content managed in a cohesive composition. These include: directors, producers, PR, music, gameplay, music, graphics, UI, writers, localization, and so on. This is not their first "rodeo." They obviously employed this wide variety of talents in the hopes of keeping this game alive for as long as they can afford to.

I see no big mystery. They are a business. They charge money to entertain us. We pay them money to keep us entertained. They provide us with new things so we stay entertained. If we stay entertained, we pay them more money. If they make more money, their investors make more money. If their investors make more money, they stay employed. If they stay employed, they can afford to develop more content to keep us entertained.

Thus the cycle continues.