A really good ending is defined by a few things, but one of the key factors is that a truly great ending doesn't have a follow-up. After all, it's an ending, the wrap-up of the entire narrative, the final ride off into the sunset. Adding more onto that would, by its very nature, ruin it - even if that new stuff is really good.
However, sometimes you get a new idea for a follow-up, or a project initially intended as a done-in-one is more successful than you anticipated, or even the story has turned into a money train that your bosses ain't gonna let stop unless they have a proverbial (and possibly literal) gun to their heads. Whatever the case, the end turns out to not be the end. This can prove embarrassing when you wrote an ending with the intent of never doing a sequel.
Some game endings are so brilliant, so final, that a sequel feels impossible to pull off. For better, and indeed for worse, these games ended on the most final note imaginable, and then just kinda kept going.
10. BioShock 2
There are many ways you can go about definitively ending a story. One of the most surefire ways is to utterly destroy the setting of the story. How much more perfect can you get, right? Can't have a story when the story has nowhere to happen. So when 2K Games demanded a sequel to BioShock, the devs decided that this would be the end, and went about it accordingly.
BioShock 2 feels needless at every step of the journey. Despite the gameplay being, in many ways, far superior to its more famous sibling, nothing about it screams that this is a sequel that needed to happen. This is probably why BioShock 2 ends with the city of Rapture being abandoned for good.
Sure, it's still technically down there, but the falls of Andrew Ryan, Frank Fontaine, and then Sofia Lamb leave the city with little to no infrastructure. The destruction of Hephaestus means very little of Rapture still has electricity, and pretty much every city official has been murdered. So yeah, safe to say Rapture ain't bouncing back. In fact, this entry is at the bottom because, technically, it didn't.
Oh sure, there was another sequel, but BioShock Infinite is more along the lines of a soft reboot, until over halfway through the game and into the DLC when it explicitly references Rapture.