Because teams have budgets and they have since signed other players
That, and if someone else was offering more than 10/300 I'm sure he would have signed by now. When Boras comes back to the Nats and says, 'Okay, he'll take the 10/300 you believe he's worth,' the team will just say, 'Believed. Believed he was worth before you tested the market and proved he isn't."