Preloading in Search (along with exclusive occurrences in Top Stories Carousel) people cite as one the most unfair advantages for AMP.
Why do you think Google doesn't do the same for some non-AMP sites?
(I'm a big fan of AMP and I know that Google has more than one reason to do it. But I wonder what the community thinks.)
Google does it on Desktop. On mobile, the stories become a carousel(in article read mode), if they sandwich non-AMP articles, the carousel may not work.
Also, it may be tough to make sure the non-AMP page will load faster and can be read easily.
However, non-AMP pages can still feature as Top Stories 'header' on mobile.
