Multi-core CPUs were still starting out to be fair, but they were definitely at least somewhat mainstream by the time of the 360/ps3. The 360 was tri-core, and was considered easier to develop for since all three of those cores shared resources. Meanwhile, the cell architecture is hard to develop for even by modern standards. As such, most games only made use of the PPE and left the SPE alone.

The problem was less parallel processing but that every one of the cell‘s 8 co-processors (SPE) needed to be individually programmed. The 360 had a tri core design that was much easier to develop for and take full advantage of. Thus, most 360 games, especially early in the generation, look and/or perform better than their ps3 counterparts, since the latter usually only ran on the one regular processor core (PPE) that the cell had, without taking Ananas off the SPEs. Notable exceptions are the ps3 exclusive titles and some other later games, that took partial or even fully advantage. Even Naughty Dog only used 3-4 SPEs in their earlier uncharted games, while their later games like the last of us uses them all.