Perhaps part of why their website is so good is because they deliver the goods. They follow through. Ever need a 4x8 sheet of copper? It gets delivered to your front door. Sooner than you expect, and at a very fair price.
MMC is mostly about not making the mistakes everybody else makes, which puts it near the top even before you consider what they do well.
They clearly are indifferent to customers except to the point required by law, or as needed to keep the cash flowing in.
It costs money to do the things that MMC does and Amazon prefers volume and revenue over the margins that MMC can command by providing quality service.
Also, my own experience was that I did not pay a "premium". I really did need 1/2 sheets of copper on a regular basis. I could get small pieces at a premium. I could get larger sheets by driving for a hour or ... I could pay about the same total cost or less and get them delivered to my door... This was not limited to a few items. I needed plastic bags, about quart size. Again I got just functional bags - not shoddy at all - at a very reasonable price. The same things at amazon would have been a) not functional or b) impossible to find among all the shoddiness.
Sad to say, comparing them to most other sites where you try (or attempt to try -- yes, often it is that bad) makes a case for saying, yes indeed, they are great!
I still check them though as they often have good enough parts for a lot less than OEM prices. Though I also still go to the OEM when I get tired of third party parts that don't work (after the switch issue above I went to my local auto parts store - the switch fits but doesn't work, I'll see what the OEM part does. Rock auto was $8, local store $20, deal $140 - but I trust the dealer switch will work for another 20 years)