The actual ISO standard falls well short of being useful/sufficient to anyone who isn't an incumbent player. It's effectively a moat and therefore a direct impediment to competition from teams who have novel technical ideas but don't have access to significant capital - building a SQL implementation is a long, expensive journey. This is why many startups resort to building Postgres extensions, or using Calcite or DataFusion.

If SQL weren't so (needlessly) complex we would see much more competition across the database space.