If you mean why does the cache (of which there are 3: google, cloudflare, and Microsoft/Bing) get to control the preloading, the amp limited set of html can be safely cached in it's entirety.

So when I visit a search engine and an amp page is preloaded, it doesn't communicate with that domain.

This ensures that cross domain preloading is possible without leaking my information to any third parties (where third parties in this case is "random sites on the first page of Google").

If link preload was used, any time you made a search, it'd be equivalent to opening all the results on the page, from a tracking perspective. That's pretty awful from the point of view of privacy.