The answer to the OP is, imho , simple but quite unexpected from Apple. It all has to do with the current limitations of iPadOS, its planned near-future features and sort of people that would buy the 1TB or 2TB iPad Pro (and thus the 16GB ram option). TL;DR version: Future Proofing it for upcoming Pro Apps!
So, currently, iPadOS 16 cannot even benefit from having 8GB. In fact, as of now the max RAM that iPad Apps can leverage is 6GB. The only "benefit" (if you can call it that) of having 8GB on it is being able to have a gazillion more safari tabs and apps open concurrently ... for whatever that's useful for.
So, because of this it is well known that 16GB is already overkill for the current iPad OS. This means that 24GB would be an even more ridiculous proposition. As it has been mentioned already on this thread, having a 2TB IPP with 24GB ram SKU would literally be a huge loss-generating move from Apple. It would be priced so ridiculously high (even more so than the ludicrous pricing of the current 2TB model) that pretty much NOBODY will buy it, but you they would still need to stock it anyway, hence losing money.
But then you say, " Fine, 24GB would be ludicrously overkill, but 16GB is also overkill and they are still making them anyway right? Why didn't they just keep the 1TB and 2TB versions at 8GB ram??...If they would have done so they would be a bit cheaper and perhaps more people would buy them, right?
Well, the answer to this question has everything to do, IMHO, with future-proofing. Ask yourself, who are the likely buyers of the 1TB and 2TB SKUs? Likely, actual PRO users, people who would actually use the IPP for a living, running DaVinci Resolve or Logic Pro (insert whatever upcoming X "pro" app here). Sure, 16GB doesn't benefit them NOW, but I think it's highly likely it might benefit them in the near future. For example, what if iPad OS 17 allows for apps to leverage 8 or 12GB of RAM? or maybe iPadOS 18? If so, these apps can be patched to make use of this. This, plus perhaps future iPadOS improvements to the stage manager and other general multitasking features might bring iPPs to a point when you could be viably running both DaVinci and Logic Pro smoothly, even more so that you could do that now.
But then the OP would say: " Sure, but if they make a 16GB version as a future-proofing, the same applies if they made a 24GB one, right?". Technically yes, but the problem is that we are so far away in time to the point when iPadOS can leverage 24GB ram (and the IPP use case scenarios that could warrant this) that when we reach this point, M1-M2 IPP would be so outdated that there wouldn't be any reason to still be using them. The same does not apply to the 16GBram ones ... I truly believe we are a mere 1 to 2 years away from reaching the point when both the iPadOS and the Pro Apps could actually benefit from it on certain IPP use-case scenarios.
Just my 0.002.