Why is data fragmentation not possible on main memory (RAM)?

Question

I am wondering why data fragmentation is a problem on main memory.

On a software level, virtual addresses are used anyway. So why can one address space not be split up into multiple segments, like a hard disk might do? I don't see how the performance would be affected, as the time needed to access memory addresses does not vary. Is this just a limit of the MMU?

In other words, my question is why does a process need continuous memory segments? For example:

Process D requests a memory block that could fit into the two free segments, if the block were split into two pieces. Why can't this be done?

It would be great if you could add a source so I could read more about this topic, if you have one.