Because XIV is an MMORPG with a subscription model that consistently drops a new expansion every 2 years, which is a very standard business model for that genre. It's also a genre which relies a lot on social connections to maintain interest and as a result a stream of steady income for the company via subscription fees, optional services, and microtransactions. They also haven't really made any missteps since relaunching the game, nothing as divisive as say... the ending to part one of FF7R. They have a formula that works and they produce more of the same. And for the most part FFXIV doesn't require expensive hardware to play.
And also, people fall in and out of MMORPGs all the time. The people I play FFXIV with today are very different to the ones I played with back in ARR.
It's a totally different situation to a AAA single-player title (that is far more expensive in terms of resources) being chunked up and spread across multiple entries which are one-time purchases. Especially when there's likely going to be a hardware generation jump in there somewhere too. I think many people will be outraged at the idea of spending $60 every 1-2 years for 10 hours of gameplay, and it will lead to more people just waiting it out for the "complete collection." If interest starts waning on those it becomes a lot harder for them to remain ROI-positive. Heck, even Ubisoft realised that frequent releases don't maintain interest which is why (among other things) they increased the wait between Assassin's Creed releases. The only titles that really manage it (the CODs and FIFAs of the world), manage it largely because of the competitive online community moving over to the newest entry and the FOMO it causes.
Like really, it was around what - March 2017 that SQEX pulled development of FFVIIR to be completely in-house following a turbulent relationship with CyberConnect 2. So the bulk of Part 1 was made in 3 years. Now that they can re-use some of the assets, and have all the engine work and systems in place, I can see them doing a Part Two of similar style in 2 years, 3 max. I'll happily wait 2-3 years for a qualitative title with hours of gameplay in it, than have something smaller rushed out every 1-1.5 years (even though I suspect the bean counters at SQEX will push for it to be as frequently as possible) to placate the impatient.