This would be easily mittigates by the keyfob using a rolling code. The attacker can record the signal, so the car will also have received it. A replay of that specific code won’t work again. That is a principle used in cheap garage door fobs for many years. So I guess keyless fobs would have at least that level of security.

Better would be a cryptographic encryption using public/private key (already done in chip cards, so common technology). Though - looking at the dumb things car manufacturers did - I wouldn’t be surprised if they didn’t use private/public keys for this.