Whereas the indexing tools do the job effortlessly.
How do they do it?
They bribe google with links.
So the thing is, google is dumb. Kinda. They won't aggressively crawl new content easily these days (because servers aren't cheap and the web is growing much faster than it did in the 2000s).
But they will regularly recrawl domains which they think are "trustworthy" and if they are frequently updated. Take BHW for example, you can google the exact title of this thread right now and see it listed.
Once the page is recrawled, googlebot will also visit all the links on the page, and if the almighty algorithms deem your link good enough, you will end up in the index.
Indexing services have sites like this. They either have a very large site like BHW which is frequently updated and subtly place your links in place or use thousands of low quality blog domains to increase their surface area. If even one of those among the thousands get recrawled (which they often do), googlebot will see your link.
This is their main strategy. Then there's shit like abusing the Indexing API, making Parasite links wherever possible, flooding comment sections of innocent wordpress blogs with linkspam with yours (and others) site urls, and other nasty stuff which can often make other webmasters puke in pain.