Boromir and his search for Rivendell

[Inziladun]

I don't think the Last Homely House or even the valley in which it lay were in sight of the Ford of Bruinen.

(Frodo) knew of nothing that would prevent (the Black Riders) from crossing as easily as he had done; and he felt that it was useless to try to escape over the long uncertain path from the Ford to the edge of Rivendell.

FOTR Flight To the Ford (emphasis added)

If Rivendell had been that near and in sight, surely Frodo would have made an effort to ride there on the horse, and why would not Elrond or Gandalf have sent more Elves to assist, or even come themselves?