Question About Right of Redemption in PA

[Jennifer T.]

In PA, mortgagees (lenders/banks) can also redeem tax delinquent properties with same redemption period as the owner. My question is, why arent they always redeeming delinquent properties (assuming most houses do have a mortgage on it) by paying a bit of tax on it? It seems like a great deal for banks just pay a few thousands in owed taxes and get a house! 

So, how does any house make it to auctions, or am I not getting something right?