You are looking at the big restaurants, and yes, I agree with you. I'm not saying that it's zero risk. But it's also a fact that restaurants are highly saturated, not to mention those small bistros and cafes. So with a limited manpower, it makes more sense to go after the "centralized" venues like hawker centres and kopitiams where you can walk in without having to notify the owner of the private restaurant. It's less intrusive. In a small restaurant setting, if the unvaxxed sees a SDA walk in, he will walk out immediately.
If I'm a SDA I will pick a few big restaurants.