Comment by GuB-42

It requires two-way communication, which makes the system more complex, with all the negatives that come with it.

Cars are not very secure by nature: they have easy to break glass windows, and are made of relatively lightweight materials. The key system just needs to match that level of security, and AFAIK, attacks on the keyfob are uncommon compared to other, less subtle techniques.

The more complex and sensitive "PKES" system, according to the article already has a challenge-response system, but it doesn't help with relay attacks.