> everyone agrees that the website is tired, ugly, and slow, but lots of people still do all their shopping from there because they're used to it and it's easier than maintaining accounts with 15 different retailers with 15 different websites
Slow compared to what? Amazon's website feels more responsive and more usable than 90% of online stores I've seen that carry orders of magnitude less variety of inventory. It loads fast, searches fast, provides for easy navigation between related products, and displays all needed information on one page without jank. Sure, it doesn't look modern, but it feels far more functional than the competition.