If Psi is a connection to something Other, something divine, then why is it so weak? Why does it end up in the hands of certain mediums who seem all to happy to commit fraud? And when we have honest mediums, why can't they manifest their abilities (or communion with spirits) at a stronger level?
As for why is the effect so weak? My take is that most humans are utterly fragmented and unintegrated beings, with our attention going in various directions. If you ever find yourself in a life threatening situation, the focus becomes very clear. The survival instinct takes over and everything else drops away. That's what I think is going on when you hear stories of mothers developing super human strength and lift cars etc. when their child is trapped underneath.
I also believe that is why some of the Eastern religious traditions don't deny psi, but see it as a later stage of development (if at all), after dealing with some of the lower personality aspects or ego.