Horizontally scalable data storage is generally available these days (cockroach, tidb, vitess, etc.)

Rearchitecting from unscalable to scalable data storage is notoriously difficult and expensive. Even the most famously competent companies and teams have struggled with that transition and invested millions and millions of dollars on it. Building on unscalable data storage when scalable data storage is readily available feels like planning for failure and making a huge bet that your product or system will never have widespread use.