The series is long past the point where others would have dropped the numbering to the point where it's sort of become part of the series identity. I mean the most popular Final Fantasy game is Final Fantasy 7. They didn't even start releasing these games in Europe before that, so people's first taste of the series over here was the 7th instalment. Can't really think of any other franchise where that's the case.
To look at it another way, Apple and Samsung seam completely fine with sticking numbers on their flagship phones despite the fact that they're already past the tenth instalment - Samsung even jumped all the way from 10 to 20 recently for seemingly no reason.
Also, numbered titles can sometimes sound a bit weird when you're not used to them being said, but once they start being used they suddenly become pretty natural. I remember when "PS4" sounded a bit weird when the PS3 was still current, and the PS5 when the PS4 was current. I'm sure a similar thing will happen for the PS6, which currently sounds a bit weird to say as well. And in the same way, "Final Fantasy 17" or "Final Fantasy 21" currently sound a bit weird, but if they eventually get to that number it will probably sound much more natural by then.