SQL is a humongous spec. Any serious project would rather piggy-back off an existing parser. Most of the interesting parts for most people is implementing backends against in-memory, disk, S3, HDFS, etc.
Also, a contributor to dolthub shared on Reddit last time this post came up that they originally wrote their own SQL frontend but gave up because it was so much to maintain and get correct. They ended up going with go-mysql-server which uses vitess.
From my perspective (as the author of OctoSQL): I need a SQL parser. Vitess has a fully fledged MySQL compatible one. I'd probably want to copy the open source vitess one and add the necessary features myself, instead of writing it from scratch. It's just a big endeavor, a few thousand lines of goyacc code and even more Go boilerplate.
I'm not saying everybody is using the same library, because the code has been heavily modified after being copied. A look at the cockroachdb parses suffices to see how much changed it is.
If you look at OctoSQL, there too have been many changes to add stuff like table valued functions.
Was it because they were the first and people just started using it or is it the best for some reason?