Because they won't make much money out of it.
Google already shares the 30% with operators even if they don't do billing, so what's the point? (from Android Market T&C: "The Transaction Fee, as defined below, will be charged on the sales price and apportioned to the Payment Processor and, if one exists, the Authorized Carrier."
The Payment processor gets 5% (Google Checkout), so the 25% goes to the carrier either way.
Carrier billing means more paper, more records, more customer complaints about charges, it's not worth for most just to get 5% of sales.