Few things before I start:
I've never seen anybody talk about this yet, so imma talk about it now.
So y'know how raids work, right? You and a group of of people fight big pokemon and when you kill it, it is now smol and weaker than their battles counterpart, and is also unable to be known as shiny until caught.
And dynamax raids are... The same, except only up to 4 instead of (I think) 16
So now tell me how raid pokemon are big and strong? We know why for dynamax, so my theory for when it happens is that all raid obtained pokemon (after the update and from then on) will be dynamax, that way, it not only introduces the feature in an opening way, but it also explains why they get weaker upon defeat.
Can't wait for game theory to see this and make it a vid or smth.