That's not how search engines work. They have a good idea of which pages might be frequently updated. That's how "news search" works, and even small startup search engines like blekko had news search.
Indeed. My understanding is that crawl is a real expense at scale so they optimize for "just enough" to catch most site update rhythms and then use other signals (like blog pings, or someone searching for a URL that's not yet crawled, etc) to selectively chase fresher content.
My experience is that a news crawl is not a big expense at scale, but so far I've only built one and inherited one. BTW No one uses blog pings, the latest hotness is IndexNow.