The exploration, and the early game in general, is very interesting, with a plethora of events, not limited to Anomalies surveying player and AI science ships can encounter.

Stellaris is steadily receiving updates, tweaking game balance and fixing bugs alongside with quality of life improvements and new content. Though some of the bigger updates content is often sold as DLC.

The fact that Stellaris is so modifiable makes a huge difference to the game. There are so many ways to customize the game to your liking.

If there's a design decision you dislike, you are always capable of changing it yourself (within certain limits), and you can make the game completely different. You can play as a race from your favorite books, anime, whatever, because so many talented people are making mods for the game.

Oftentimes, if you play the game without min-maxing everything, you will find yourself with interesting stories to tell. While the coded events themselves may lose their touch after a while, the interactions with the various races nearby will often be different. You might find yourself in situations where the whole galaxy is against you. You also might find a race of primitives who you uplift, who will become fast friends with you. It's possible that you might grow attached to them, only for them to suddenly and violently be taken over by that Xenophobic Militarist on your borders.

The reason some stick with this game for so long is because they love these interactions, despite how shallow they may be sometimes. The fact that you can even be a Fanatic Militarist in the game and still have friends afterward is something that some may find particularly fun.