Besides the oversaturation with spammy ads (are they really ads if they're for, and on, their own platform?) and Prime nagging,
The search is just utterly braindead.
I can maybe believe it's optimized for consumer items somehow, but I don't shop for those online. At least not from them. I'm usually searching for something technical, and it takes easily half an hour to sift through the results, try different keywords, discover new keywords and categories, and finally read a few (sketchy) descriptions to have reasonable confidence in what I'm about to push the button on.
Meanwhile, I've easily spent the labor of buying the thing straight even from McMaster (which, if you don't know, is one of the highest marked-up distributors you'll see, among engineering-related distributors I think). Let alone DK/Mouser, no contest (their markup isn't as aggressive as McMaster, I'd say).
Tim