Because we can't out bid the Mets in free agency and these things don't happen in a vacuum. If they say it's 400m now and we tell Boras we will go to it, the Mets could say okay we'll go $450m next year if you wait, but with the possibility of a long/potentially season ending work stoppage to come, it makes plenty of sense to say I'll take the guaranteed money right now, especially for the enormous figure it'll be. And if there is a year long stoppage of even half the season, who says he gets anywhere near that figure again. I'd rather we tie him up now and pay the prosect price rather than the alternative.
Again though it all feels very unlikely, and I don't expect us to go there.