I thought about this recently - probably Valve is reluctant to because people are still licensing both the GoldSource and Source engines (I think the last GoldSource engine license was in 2020 but the game hasn't even been announced; I think four Source engine games are due this year). If EA released the Nightfire engine code (very advanced GoldSource engine fork) that would be great though!
But it's sad that John Carmack's example has not been widely followed.
I think it's more to protect the current licensees though - I have no idea how much an engine license costs, but I don't think it's cheap, and I'd be pretty annoyed if I'd dropped that money a year or two ago only to find out that the engine costs nothing.
That said, I'd be a little surprised if any more engine licenses to get sold so maybe give it a couple of years.
I know! I played some multiplayer (against bots) not long ago and I don't remember it from the first time around, but there were no blood decals. I wonder if Gearbox just developed one low-violence version that they could sell in all markets.
But it's sad that John Carmack's example has not been widely followed.