Because if it doesn't there won't be one; as israel clearly would prefer to hold onto all the land it has stolen, and the Palestinians on their own have little leverage to force that to change. they need the help of the world. Although a settlement based on the 1949/67 lines would in my opinion be fairest, and legally most correct, the US clearly isn't going to try and impose that at all. There will be some land swaps. The rest of the broad detail we also know. If Israel doesn't want the solution imposed, best it negotiates now in good faith and with urgency.
UN: Gaza could be 'uninhabitable' by 2020 if trends continue (AP)
from the article: Let’s stop pretending