I thought the same thing years ago, but you have to compare apples with apples and not apples with oranges. Google does not need SEO… 😊 and Amazon and other huge sites will get indexed as you have seen but still might not get the best search results.
Um, if you take two smaller, newer sites, with pretty much the same content and meta rich keywords, the valid site will most likely get better indexing in search results.
There is an article from google folks (that I should search for) that sort of says that sites with valid markup will be taken more seriously then sites with hack markup..etc.
Anywho…long story short…don’t bog down it perfect validation but try to do all you can…it’s worked for myself and many others…this really is a old debate that’s been around for a while…
but I like you was upset one day when I saw my hero “Google” hehe…with the worst markup and hack bastardized inline css…js…etc…lol.