The TLDR is that MMAP sorta does what you want, but DBMSes need more control over how/when data is paged in/out of memory. Without this extra control, there can

The TLDR is that MMAP sorta does what you want, but DBMSes need more control over how/when data is paged in/out of memory. Without this extra control, there can be issues with transactional safety, and performance.