It’s partly a design choice i agree, but let’s breakdown what happens during startup of the sim.
After i click the shortcut it takes 15 seconds to show something on screen (Steam version here), which i honestly don’t understand why.
Then the client starts up during the splash screens. This is the client that is downloaded through the Microsoft or Steam store, this isn’t the content, it’s only the client that manages the updates and the content that resides in packages (the 130GB or something packages you download from within the sim). They chose for this solution probably because of the package management which wouldn’t work like this with both Microsoft Store and Steam and their cloud solution.
Now it logs you into your Microsoft account and checks for updates. This is the “Set your experience” part with the turning wheel in the middle.
Then it moves to the part with the loading bar, the actual loading of the game. Packages are loaded/checked whatever (everything you see in the content manager in the game, not only community folder).
All these steps don’t take that long individually, but the combination of them do because they chose for this model of package management and a very basic client (the thing that gets updated by steam or the Microsoft Store).
They chose this and i doubt they will redesign the sim from scratch as they chose this philosophy for updating the sim and it would require a complete refactor.