Because the next level up is HHonors Gold, and that has real value (free breakfast and free internet at all properties worldwide on all days of the week).

Just for reference: Holding the Priortity Club Select Visa from Chase (which has a "net zero" annual fee (ie, although it has a $49 annual fee, for anyone who ever needs hotel stays, the unrestricted free night certificate you get each year you pay the annual fee should more than make up for the annual fee), gives you Priority Club Platinum status (the second and btw highest level in that program) as long as you hold the card.

Prioity Club Platinum is better than HHonors Silver (at some hotels it gives you free breakfast and/or free interenet), but it's not as good as HHonors Gold (the free breakfast at Priority Club hotels is often only a subset of the breakfast, and at any rate you have to research which hotels even give it and/or free internet to PC Plats, because only a fraction of PC properties do).