I literally asked my son this exact question when he threw a huge tantrum over his birthday when he didn't get a Nintendo Switch. My exact words were "How in the HELL are you even slightly surprised by this?" After spending six months constantly bugging me about it and having me so "No" literally every single time it came up, he still somehow assumed I would get it for him anyway.
It's called "wishful thinking." It's how you set yourself up for false expectations when a thing you WISH would happen begins to seem more and more likely TO happen in your mind despite all evidence tot he contrary. "I wish I could get a promotion and make more money!" becomes "I'm going to get that promotion and make more money!" Then you get passed over for the promotion because your performance is poor, you're consistently late, you don't take direction well and you're generally unqualified for the job. You're disappointed, and you're even a little angry, but did your boss actually screw you over or did you just get your delusions shattered in the most abrupt way? (I have literally been on BOTH sides of that conversation at least twice).
Because people spend more time talking among themselves and/or talking TO themselves about these stories than actually watching the stories. A fairly large amount of the lore around Star Wars and Star Trek doesn't actually come from canon at all and is derived from fan theories and audience assumptions about what's really going on.
As a really glaring example: a tremendous number of Star Trek fans believed (and many still believe) that Spock was the only Vulcan in Starfleet during TOS. This is an interesting interpretation of what we saw of the fleet from the old series, but there's nothing on screen that actually establishes that this is the case. But someone who spends a lot of time pondering the implications of that theory -- and becomes very familiar with it and internalizes it as part of their vision of who and what Spock actually is -- would find it very hard to adapt to a version of Spock where he ISN'T the only Vulcan in the fleet, or even the first, or even particularly unique in that regard. It's someone who expects something to be true about Spock that was never actually true in the first place and then becomes angry when it turns out it isn't.
People have a right to be childish and unrealistic. But it is what it is, whether you choose to defend it or not.
Pretty much my whole point and differentiation from your scenario comes down to the parent not saying no...I.e the producers repeatedly saying ‘prime trek’ and also implying it. Do you see?
Expectation raised a party other than the individual, by the person (S) in control of delivery...yes?