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The short answer is game balance, the game's intended difficulty balance is built around it always being active. Whether that is your desired difficulty doesn't factor in, it's a children's game after all.
But that doesn't explain the 'what' that has changed between the older games that didn't have exp all and the ones that do now. So, the ACTUAL reason for it is the amount of pokemon they drown you with early on. Typically, you would find about up to a dozen pokemon before the first gym, but in SV you encounter nearly 20, and you have barely started the game yet.
The fact that you can access the PC anywhere, the exp all, and the sheer amount of early available shows that the design 'intent' is for you to be constantly switching in and trying new pokemon. Unfortunately, GF don't seem to get that most players prefer to have basically the same team the whole game.
Personally, I don't mind the exp sharing. The shared exp is so minuscule it doesn't really move the needle. Though the philosophy of vomiting pokemon at you so early has it's issues. I believe an RPG staple is 'limitation breeds creativity', giving you basic tools then slowly opening up. Frontloading content just leads to burning people out.
