First of all, the number of different dialects is a non-issue because projects that have to send the same queries to different databases are very rare.
Second, in my world where performance matters, you need to hand optimize queries. This is done quite specifically based on the database load and size, let alone specific quirks and strengths of the actual DB product. Something like this would only get in the way.
Third, even if all of these aren't important to you, and you just want to avoid SQL because you don't like it, there are plenty of ORMs in circulation. Many of them allow to specify queries in a fluent fashion.
Finally I think the cardinal issue is expecting that inventing a new dialect will change anything -- imagine the PRQL that compiles to a hundred-lines complex SQL query: would it be necessarily more readable? I doubt it.