Because they won't give you the remaining 3/4. And you've wasted months, and destroyed your support among the public by trying to negotiate with these bastids. People need help now.
Or, you pass the entire thing through reconciliation. And then pass another huge thing that helps people with your second available reconciliation option.
I'm with you here. The current iteration of the Republican Party has no interest or inclination to negotiate. Sure, maybe you'll get 1-2 of them per house to work with you in something approaching good faith, but so what? The rest won't follow in the best scenario, and will just Lucy the football in the worst.
Use the hammer of reconciliation. Then use it again.