From Amazon's perspective it's the same and worth optimizing

Whoa whoa whoa. Amazon has focused on page speed load and user experience for over a decade. Their core business is their website. They have refined and improved thousands of times. You should ask "What would happen to Amazon's sales if they suddenly stopped caring about page speed?" The answer is over time they will add additional bloaty libraries and it will eventually slow down to a horrible crawl. This will likely start turning customers away who want a better user experience.

On a daily basis I have to interact with HPE's website. Now THAT's a company that could care less about their user's experience on their website. Pages take on average 20 seconds to load and sometimes up to 3 minutes! They are lucky that it's not their website that is their business but it's their products or else they'd lose business so fast. I absolutely HATE interacting with their site because of how slow it is. I can deal with bad UX if the site is quick to load, but I can't deal with bad UX if the site is extremely slow.

Another thing to think about is simple single content pages, like ones that just tell you the time, or tell you what your IP address is. The ones at the top of Google searches pay extremely close attention to page speed load times. There is so much competition in their space that they know they need to be the best and that is just another way to do that.