I'm not working on THEIR machines - I'm configuring their account on MY machine. The customer isn't even present - they send me their details by phone or whatsapp or email.
Usually I can do it by TeamViewer, but not everybody likes having that installed.
If you have clutter on your computer there is only one person to blame.
Like I said - it is my preference.
I'm not going to sign out of Facebook on my own browser when I can just as easily sign in on a different browser.
If you don't like that... tough tatties.
There are multiple reasons for these things to exist.
Some of these browsers have tighter security, some of them employ multiple layout engines, some of them have features that other browsers lack.
If you want to remain ignorant of that - it's not my problem.
But I love to test and play with software so that I can recommend it to other people and know what I'm talking about.