Is it?

Hotel room = CASH
it comes with food vouchers = CASH
First Class seat = an upgrade for another (presumably better) customer

VDB voucher = funny money, a percentage of which is lost, forfeited, etc., so it doesn't cost the company nearly its face value.

I've seen other airlines try to solicit locals who don't need hotels, because that is great dealing. That saves the company money. If they have to up the voucher to avoid paying for a hotel, that's smart business, because the voucher isn't worth much, but the hotel is.

Further, a voucher is a driver for repeat business. For example, if you're taking a trip, and the lowest fare is $500, but CO is $600, but you have a $200 voucher, well, you may take CO because it's net cheaper after your voucher is applied. So CO gets $400 when it would have otherwise received $0. Had this customer had a $100 voucher, it wouldn't make sense to use a voucher and pay what you can otherwise pay on a competitor without burning the voucher.

Finally, throwing a first class seat at some college student just to save another $100 in voucher is probably net more costly to the company. The upgrade situation out of SFO is particularly tight, so giving away such a seat in order to save a few bucks in voucher (which probably isn't worth even $50 in cash) at the expense of the customer experience of a frequent flyer who spends thousands of dollars flying 75,000 miles a year or more, is very short-sighted.

If First Class were wide open, it'd be a different story. If the bumped pax is Elite, sure. If F were all that were available, fine. But out of all stations, for SFO to be giving away what limited F capacity it has for such a trivial savings in voucher, is likely more costly for the company in the end.