Google keeps looking for 404 pages

Googlebot requests repeatedly, over a period of months, some nonexistent
pages from a site I manage for which 404 responses are returned.

I used the link: feature to find out what pages link to the missing
pages on my site, and no hits are returned.

So if Google keeps getting 404s, and it isn't finding links elsewhere
that give it reason to think those pages should exist, then why does it
continue to look for these pages?