Is it real, or just my imagination,
But I could swear, most important projects (cancer, AI,...) all give out less PPD than projects that matter less (like Prime numbers, or rake search).

Is there a reason for this?

Also, I'm surprised a lot of important projects (like World Community Grid, QuChemPedIA, and others) haven't shifted to GPU.
They're still running on slower CPU cycles...

It's not a case of haven't moved on to GPUs as not ever project has either the required knowledge or people to write the applications for GPUs. This particularly applies to jobs that are not going to be around for a long time which is typical of those running on WCG.