"Not necessary" is not the right term, is not "required" to have a box, but I would still prefer a back box personally. I would say they are not designed with a box being required, as mentioned, not for performance but out of lack of consumer level knowledge. I think the reason most do not have them is their use case isn't designed around bass extension and back wave control. The main criteria being: fits in a typical wall/ceiling and not including a box means the cost is not passed on to the customer, though you would think these companies might want to offer it as an add-on for additional $$
These are designed to be convenient, not perfect. For those who want to go the extra distance, a box can be built or cobbled together but the drivers are made for infinite baffle mounting unless otherwise specified. Some end users or integrators won't bother, some will and for them, it's not hard to come up with something that isolates one side from the other but for those who want better, speakers that come with a back box is another option. Focal makes good speakers, Dynaudio is another brand whose speakers have a really good reputation and even their in-wall and in-ceiling speakers come without a back box but the main reason is that it's expensive to ship with it, so they leave it to someone else. I only installed one pair of Dynaudio in-ceiling speakers because most people won't spend that much, but I did build back-boxes because these produce fairly strong bass and if that had intruded into the living space above, it would have been a problem. I will say, though- they're the best sounding in ceiling speakers I have heard, bar none. OTOH, the best in wall were from Triad and they DO come with a back box, at least the ones we installed.
I'm not sure the manufacturers would be interested in offering back boxes, but they could experiment and find a good size, then make the diagrams available. Not knowing the correct volume is one thing, not making it available is a disservice, IMO.