Google has been trying to dumb down their services and products for years, to the point where the UI is something even a baby or old person with dementia can use it. For the most recent example of this see the latest update to the YouTube app for iPhone - they managed to make an already austere interface completely bare-bones, to the point where the main "home page" is entirely useless.
They do this to product after product, all the while claiming things like "power users are not our target user group" or "only 5% of users used this feature, so we removed it", and for random millenials using their phone to figure out what bar they want to go to tonight, that approach works (ish).
But Calendar is actually used by lots of working people/professionals/power users every day, and those people aren't interested in a super stripped-down "beautiful, magical" (also inscrutable) interface that rips out every feature that's not used by at least 90% of the userbase for the sake of whitespace. We need a tool that is predictable, works, and doesn't require retraining for everyone to use it.
So Calendar stays frozen in amber, caught between one pole (being actually useful) that Google desperately does not want to migrate towards, and another pole (beautiful, magic, useless) that Google desperately wants to move to but knows would alienate their userbase if they did. And while normally they give 0 fucks about that, because of GAFYD they can't just wave away the consequences of such a change - that business unit would be in serious danger if they managed the apps & services within it as capriciously as they do everything else.