Website data store doesn't not index most of available website documents

[kcd]

Hi,

I am building a search app and I created a website data store using something like "mysite.com/*".

But only 3259 (out of ~80000 pages) are indexed and available to search for.

I have found a lot of cases when such a page is marked as "indexed" in google search console and is actually searchable by google search but is not included into "search index" of my application.