You don’t need technical knowledge to see the problem.

If you live in an apartment and your landlord has a master key, then all an attacker needs to do is get that master key. In an apartment complex, maybe that’s okay because who’s going to break in to the landlord’s office? But on the internet, tons of people are trying to break in every day, and eventually someone will get the key.

Even for the landlord, I’d rather them have a copy of my key than a master key, because that way they’d need to steal my key specifically.