That's partially because people see PALnet as double-dipping (extra content rewards there on top of the content rewards here).
But you have a good point, and in fact PALnet rewards will ultimately become dominated by accounts that have substantially more PAL than SP.
Because they are blind. Inflation steem 8.x% + Inflation PALnet 10.x% + 2.x% in/out = 20.x%
PAL has its own supply and it is not a factor of steem supply, so the inflation is not additive.
If you hold both, your inflation devaluation would be a weighted average, depending on the amount you hold of each, not a sum.
PAL is paired to STEEM.
It trades against STEEM and requires using steem as gateway, but it is not paired to it.
To say PAL is paired to STEEM is like saying EOS is paired to eth, just because that's where the ico was.
/facepalm