It isn't memory utilization it is bandwidth. The CPU can only get so many bytes in and out from main memory and only has so much cache. Eventually the cores are fighting each other for access to the main memory they need. There is plenty of memory in the system, the CPU just can't get at enough of it.

NUMA (non-unifrom memory access - basically give each CPU a serpate bank of RAM, and if you need something that is in the other bank of RAM you need to ask the other CPU) exists because of this. I don't have access to a NUMA to see how they compare. My understanding (which could be wrong) is OS designers are still trying to figure out how to use them well, and they are not expected to do well for all problems.