You've got to have a line of separation between what's a non-pro device and a pro device - both pricing and hardware, and RAM easily falls into that.
Also, saying Apple should have put more RAM in, for something that was still being developed, is daft. When the iPhone 15 range was being designed and prototyped/finalised, would have been very early last year. We have no way to know for sure, but I doubt they would have had Apple Intelligence working enough to fully realise how much RAM, along with the processing chip power needed, to run Apple Intelligence in a way they were happy with. Seeing as it will be labelled as 'beta' when iOS 18 launches in the fall, that further adds more to my point.
Don't get me wrong, Apple, and other companies, will have limited things in certain versions of their devices, but that's to different audiences, with different budgets, etc. That's why you get Pro/Ultra/'a' models, etc. All companies have sale tactics, some are dicks, some just get portrayed as dicks for people that wanna gripe.
I wouldn't be surprised if the full iOS 16 lineup has enough RAM though.