A simple example is this: A couple do online grocery shopping every week or so, depending who has time to do it, one them will log into the 'account' and build the basket. Maybe the other will then amend the basket a few hours later before the cut off time. With enforced MFA, this is not possible.
There will always be a small percentage of situations where '1 person = 1 account' will never be true. Until providers add the concept of multi-logins to the same 'account' on their systems you can't wholesale move to stronger security methods.
I have the same issue with all these 'smart home' products that need an app installed onto a phone or tablet. A lot of them are bound to a single account, which means if other people in the household also want to have the app, you have to share your account details. if it's a Google or Amazon product, that means you are sharing account details of an account that you really shouldn't be.