I definitely lean more toward NIH than what's conventionally considered wise, but most of the time it's not NIH for the sake of NIH.
I do pull a lot of libraries, but an enormous amount of what the search engine does is very much built from scratch. The libraries generally deal with parsing common formats, compression, serialization, and various service glue like dependency injection. I think the number of explicit dependencies is a bit inflated by the choice to not use a framework like springboot, which pulls many of the same (or equivalent ones) implicitly.
What makes the search engine a search engine, the indexing software (all the way down to database primitives like btrees etc.), a large chunk of the language processing, and so forth; that's all bespoke. I think it needs to be. A lot of existing code just doesn't scale, or has too many customizations that would add unnecessary glue and complexity to my own code.
I'm going to echo SerenityOS Andreas and suggest that it's a skill like any other. If you shy away from building custom solutions to hard problems, you will never be good at it; and it will become a self-fulfilling prophecy that these NIH solutions are too hard to build.
At the same time, there's a time and a place and you should indeed be judicious as to when to roll your own solutions, but maybe that time and place is exactly in a hobby project like the ones suggested in this thread (and is how my search engine started out; a place to dick around with difficult problems).
I'd also add that being able to tackle problems yourself, rather than needing a library to do all the heavy lifting at all times, is a great enabler. Sometimes there is no adequate library, but that doesn't mean the conclusion has to be "welp, I guess we can't do that yet..."