I like to see & try to choose my fish carefully, you don't get that with online sales except in very rare cases. I also like to see the health of fish kept with the 1s I want or on the same filtration system. Learning what healthy & sick fish look like is an important part of our hobby. Many new fish keepers give up after a major die off. That's understandable, it can be heartbreaking.
It's not so much paranoia as experience. I don't want to unnecessarily have the expense, worry or work involved with diseased fish. Some things are easily treated if caught early, some are not. I have a few fish I will not risk...& some I might for the right new fish...but I also almost always QT....that's sometimes the price of past experiences, & having lots of tanks for rare or unusual species.
Amaz, your new to AC, we have no way of knowing your level in our hobby, newb or world class breeder & there all the in betweens...(snarky comment deleted) Even before the internet (OMG!!) there were encyclopediae of fish we drooled over even if we hadn't much of a chance keeping them well...but we wanted to!
Good luck!