Cuz family!/s
In all seriousness, plenty of movie critics have called them out on this, but many fans and casual audiences don't care to listen. Ironic enough, there was a bit of backlash with 9 on the audience's part. The majority of critics actually enjoyed part 9. Not to mention, most critics didn't have a problem with part 5 or part 7. 7 seven is a bit more understandable though.
See, I don't have the time (or care factor) to get as granular as assigning merit to the individual films like that, let alone what care critics think. Once you hit a 4th installment, it's apparent you're cashing in on name recognition and just doing it because people will pay to see more. Which I suppose is a sound business model; I mean, why stop when the audience won't, right? But it just harkens back to the collective complacency of said audience. Work smarter, not harder: why try when they don't expect us to? (Looking at you, Matrix 4...)
And this is coming from a massive fan of
Ocean's 11. I LOVE the first movie. Then
Ocean's 12 came out came out, and I was "oh, okay; could be pretty fun." Then Ocean's 13, and I was "... okay, let's not get too ahead of ourselves; you've already made some solid flicks with solid endings." Then
Ocean's 8 came out which I couldn't be bothered to watch because clearly they were cashing in. I think I might have watched it in a drunken stupor, but nothing about it was memorable enough to bother using valuable memory space in my noggin.
TL;DR? If you can't get through a story in 3 films or less, you're not making a story; you're just trying to make money, and I have more self-respect than that.