That's also so they can deny refunds for cash balances — once you pay money to their company, you may never receive it back under any circumstances. That way they can 'refund' you games at any time, but not ever have to record a cash transaction to do so. This is, not coincidentally, universal among online video game and microtransaction stores, with the singular exception of Apple who does not compel all purchases through an intermediate currency balance.
Steam only requires the Steam Wallet for microtransactions. Games and DLC are usually bought "directly". Refunds go to the original payment method, unless you explicitly request that it should go to the wallet.
> with the singular exception of Apple who does not compel all purchases through an intermediate currency balance.
Wow, I just realized why Steam does this for in-game purchases.