Main memory is rarely encrypted, unless you have special security features in your CPU to do so. Only the disk is encrypted; main memory is vulnerable while running. Also see https://en.wikipedia.org/wiki/Cold_boot_attack

So you don't want any hardware to have access to main memory if it doesn't need to. For instance, you can use an IOMMU to ensure that devices can only access the specific areas the OS wants to allow them to DMA to/from, not all of memory.

> What's the point of accessing main memory in a locked and encrypted phone?

The phone isn't always locked and encrypted; for example, whenever the user is using the phone it's unlocked and decrypted.