Because it works great as a platform, and because I've been using it for 20 years.
But for a new user, it looks completely messy, with pages that are vastly different from each other and many sections that look exactly as they were in the early 2000's.
I'm not sure 'it looks like it did in 2000' is worth tearing it all down, starting over, and introducing new bugs. Sometimes things just work. Ebay is very functional for a huge amount of commerce. It loads reasonably quickly and is reasonably easy to navigate.