Why routers don't just use Diffie-Hellman protocol? Routers could just use Diffie-Hellman protocol with any computer who tries to connect to that router. Then it would be impossible to eavesdrop for anyone. Only router and computer would know the secret key for that session. No?

Or someone could act as a router and send his own public key and get the traffic through his computer and that is the reason routers don't use it?
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.