Because theoretically when holding 1 PIVX in a wallet, the chance of ever getting a block reward is so small that it probably won't happen for a year / few years. According to the calculator it would take approximately 40 years.

Getting a smaller reward on short term might therefore be a more substantial form of interest (however very very little in USD). Because let's say someone is 30 years or older, the chance of dying before getting the block reward is not unimaginable. And by pooling PIVX, that person gets 0.01 USD of interest maybe, a very small amount not really worth the investment, but that always beats 0.00 USD interest.