The problem with that is that you're pushing the resource management problem onto the administrator (who now has to statically partition their system's resource

The problem with that is that you're pushing the resource management problem onto the administrator (who now has to statically partition their system's resources) and the database (which has to reimplement a lot of the resource management that the OS would do, like I/O management and I/O caching). There are many reasons why this is not a good trade-off for the vast majority of use cases. Unless you are running something like a traditional Oracle deployment where you're just giving over the entire host to the database anyway and the database is enough of a cash cow that they can afford to re-implement much of the functionality of an operating system. It generally makes a lot more sense to let the OS manage the buffer pool using the available free memory on the system (this is what Postgres does, for example). It also makes more sense for the vast majority of deployments to allow process memory consumption to be dynamic and deploy the appropriate limits and monitoring to keep things healthy - e.g. by terminating malfunctioning processes and reconfiguring and migrating applications as needed. This doesn't meet some traditional ideals about how programs should be written, but actually seems to result in better systems in practice.