Amazon is not the only search-for-something-to-buy-company. Amazon and Google can't both have a monopoly over the same market.

There's a lot of stuff that's allowed when you're a normal company and forbidden when you have a monopoly. Some courts declared google a monopoly, and that's why they are restricted in what they can display on their search pages.

(not trying to pass judgement here as to what either company should be allowed to do, just saying that's why it is the way it is.)