I even work(ed) on a game that was actually embroiled in a scandal regarding E3.
I can probably shed a little insight as to why:
1) Once you show gameplay footage, you’ve promised. This isn’t necessarily the worst thing in the world but a lot of the time the “promise” is a mega polished version of what you want to show, it’s on rails, it’s going to show all the best features of the engine, you control the pacing etc. Even if those things make it into the game (as, my game actually was in 100% parity graphically, but we ran out of budget for animations) then people /perceive/ it worse, regardless.
2) what is shown isn’t actually a game- even if it looks like one. It’s a prototype. And there’s many reasons that the final product has to be trimmed down. Either it’s required because the budget goes towards something else (such as smarter NPCs, or better animations) or it wasn’t ever possible to produce what was shown due to things that were not known at the time of creating the demo or footage. The fact is simply that you’re showing something that may not be able to exist. So that is in direct violation of my first point.
3) we got burned the last console generation, we thought the machines were going to be at least twice as powerful as they ended up being. (You May notice that my publishers “downgrade” scandal happened post-E3:2013). What we had prepared was geared towards significantly more powerful hardware. (So, number of props on screen can’t be as high, number of light sources can’t be as high, issues with streaming “pop-in”) My publisher won’t make this mistake again and with a new console generation around the corner it doesn’t makes sense to underpromise (and use the current gen, which our competitors may not have done this E3) or overpromise, and guess what the next console gen will have.
As an aside: I know it’s a trope that “console gaming holds gaming back”, and there’s a grain of truth there, but I would caution allowing developers to use as much hardware as they can, our work machines have obscene hardware (latest high end Xeon with 64/128g Ram and a gtx2070 is common), if we were not optimising for a limited platform then pc would suffer too, I just don’t know how much it is currently limited.