Crusty Old Meatwad (he/him)
I think the at-will cantrips make it seem more "Potterverse", more than anything else.
It's driven, I expect, by the McDonald's Effect.
People like to play casters (because, magic - it's a fantasy game after all), and as such they want their character to be magical, and that means being able to cast spells. As the default response to most things, from attacking monsters to locked doors to stubborn shopkeepers.
In previous editions, written in a different time with different assumptions and expectations of the real world to today, the concept of 'serving your time' as a fragile one/two shot caster with a Saturday night special crossbow as protection was acceptable, because paying your dues and then reaping rewards of your investment was worth it, and an accepted 'thing', culturally.
However, nowadays, people - especially the younger generations, but including us older folks who've got used to it, expect everything pretty much on demand - coffee just the esoteric way we like it, food fast and plentiful and ready to eat on the go in a much less formal way than before (hence the McDonald's Effect). As time has gone on we expect other things on tap too - wifi, your OS to boot in a moment, instant communications via email rather than waiting for the post, instant gratification via Likes rather than waiting to tell your mates that cracking joke you thought of the next time you see them, etc.
And so, when we play a game, having stopped by en route to grab a quick burger and fries and a cappufrappumoccachino with unicorn milk and chocolate dusting sustainably handmade by blind Gabonese virgins, and check our phone to say hi to our best friend who is shark hugging on a boat 100 miles off the coast of Vanuatu, why would we then sit down at a table and be happy to play a fantasy character who can cast one magic missile before quivering in fear for the rest of the session?
No, we want to cast spells with as little consideration as we send texts. Because the real world allows us to fulfil so many things from the get go, the fantasy world we choose to inhabit should provide the same freedom.
Before the inevitable hairshirtists declare their preference for the older way of doing things, if that's your preference, fine. But OP asked a more metaphysical why. Games reflect the culture they serve. As such, the McDonalds Effect.
I'm just off to make myself one of those cappufrappumoccachinos.
I appreciate the analysis and for the most part agree....but it's a terrible name for it. Believe it or not, McDonalds was MORE popular when AD&D 1e was at it's peak, than it is today, on a per-capita basis. McDonalds is failing right now (having peaked in the late 1990s), and it was crushing the competition in the 1980s.