not because it doesn't make sense (I'm not sure what the rationale is)
Pushing interest in the generation of console out further.
We had this unprecedented time line with the 7th Gen (PS3/XB360) where they lasted for nearly a decade. But that was the exception, not the rule. A typical generation is 3-5 years. But if you can do a hardware refresh in the middle, you can extend that out a bit. 8th Gen (PS4) saw 7 years, with the Pro releasing smack in the middle of that.
The major benefit of extended a generation -- you get the same software APIs, QA processes, support channels, and you get the benefit of mature logistics channels and manufacturing -- all of those pesky overhead costs that are hard to quantify before you jump in the deep end. Gearing up for a new generational release is difficult, but if the sales on your existing generation are dead, you have no other choice.
A new generation does give the potential for marketing, and it's one of the only ways to drive repeat sales (people who already own a console). But hey, so does a mid-generational refresh, without a lot of that extra overhead!
Same reason AMD doesn't have the 7800X3D available right now --- they certainly could, but they can wait a bit, get all those people who are itching to upgrade now to upgrade now, and then sell them another chip later on when they go to upgrade again. They get the benefit of being first to market with the latest generation, and with the mid-generation refresh get a chance to react and counter to anything the competition may have done while also garnering additional sales from upgraders.