They could put it in the back. Not all cars have airbag sensors in remote locations. Some just have them in the airbag control unit.
Yes, the airbags need to deploy early on in a crash before the occupants move too close to the airbags. Putting airbag sensors up in the front, one on each side, somewhere behind the bumper as many vehicles do must help determine if there is a real accident instead of running in to snow or barrels or something where the airbags shouldn’t deploy. But doing this isn’t mandatory. Putting a sensor in such a soft spot in the grille is going way too far in my opinion. I would not buy a car where a deer hit that smashed my hood would deploy the airbag.
Yes it is better. I see nothing wrong with airbag systems which have all of the front impact sensors contained within the airbag control unit. But I’m willing to learn more about why some designs do it this way versus having sensors behind the bumper.
The 1992-1996 Camry puts them under the front of the upper radiator support on each side. If that area gets smashed the car is probably totaled anyway and the accident is more than a fender bender. The next Camry put them on top of the frame rails just behind the bumper mounts. Seems a low speed crash that under rode someone’s rear bumper would either deploy the airbags too soon, or worse it would dislodge the airbag sensors and prevent them from working in a secondary collision accident.