Price and selection. You may love what Amazon offers today. I think they're moderately terrible, even compared to the Amazon of 10 years ago, and I have fewer alternative marketplaces to choose from and products in those marketplaces. Try finding the exact thing/seller/manufacturer you're looking for... for some things I find it damn near impossible due to how they now consolidate listings. I'm also pretty sure that the prices aren't as good as they would in a more competitive environment but can't be certain as Amazon has wiped out much of it. Competition is a key mechanism that allows markets to self regulate.

The vast majority of companies are incapable of self regulating because it is contrary to their primary mission of 'maximizing shareholder value'. This generally just means eliminate your competition and then maximizing profit. In the absence of competitive markets, the government often needs to step in at some point.