In safety-critical systems where invalid data could kill someone, using a typeless, schema-less system with nonstandard language conventions as a starting point seems irresponsible. Why not start with a normalized relational database and a language designed to encapsulate and protect data from inadvertent data-entry or programming errors?
Because performance. Yes, Epic had to put extra work in to essentially build their own version of the SQL server black box, but now they are very difficult to compete with.
Epic doesn't use the Cache SQL API. Instead, they built an API on top of objectscript that stores fields safely. They have a staggering amount of code written in objectscript, and while it's true that "nothing will stop you" from writing directly to the underlying data oddly enough that doesn't seem to be an issue because developers can learn how to follow best practices.
People are always available for work in the past tense.