Partnership networks only pays in fiat and it doesn't looks that situation may change soon. Maybe Google don't want to support integration of bitcoin as payment option.
Partners are separate, though, and I doubt YT has any say in that. Basically, YT pays them in fiat, they then pay you in BTC. Most are just afraid to get involved because BTC still has a negative stigma attached.
Either way, I'm surprised that more of them haven't embraced bitcoin. Surely it isn't just the one or two that think beyond that stigma and realize how many people are looking for that bitcoin payment option. It's also no longer as difficult to do so as it was a few years back, which hopefully they know.