Why routers don't just use Diffie-Hellman protocol?
Even if you were doing that you would only ensure that the communication between you and "some" router is secure. It's still possible to MITM using arpspoof for instance such that in:

[you] &lt;--- A ---&gt; [hacker] &lt;--- B ---&gt; [router]


Communications A &amp; B are encrypted, yet you're not talking to the real router.