I guess not all search is profitable, perhaps there gradually "optimising" on financially profitable search, so only crawling sites that relates to content for users that bring then in money, only returning SERP items that bring in money? Obviously they need their search to be broadly useful too, but many searches probably have no financial benefit for them??

It can't be for lack of crawling - my little ad-free forum which hasn't seen more than a couple concurrent users for years is crawled daily by Googlebot.

You can go in the google searchmaster’s console and tell them to scrape you less frequently, save some bandwidth.