It's about power efficiency, not total power draw. Use less powerful fans with an inefficient setup and you're not cleaning your air as well. Engineers put thought into chassis design and motor/fan specs.

For this same reason I decided to get a used box from a reputable, long lasting company with top of the line CADR, and I can find plenty of aftermarket HEPA filters that are compatible.

That was a comparison was roughly for the same air cleaning rate. I think it has to do with the fan architecture. With purpose built purifiers using static pressure fans instead of air fans.