[lsaferite]

Timing.

Proxying the radio signal over this link introduces a req/res delay. The handshake starts when the car detects fob proximity but there is still a communication with the key for authentication (otherwise you could have a replay attack). So if the car side is programmed to be strict about req/res timing you can defeat a proxy like this (in theory at least) at the expense of a higher false-negative rate.