It is a bit of a loophole, but there are some physical limitations to what you can build as a sail. Take the frac rig example. You cannot have a fixed stay going to the masthead, so you are running a high tech line up the luff of the sail. It is difficult to get/maintain enough luff tension to keep a proper flying shape. Halyard locks help, but add other complexities. On a modern race boat you go from a 5-7dg sheeting angle to maybe 12? Not ideal for going upwind. A 75% midgirth is a lot of roach to try and support, this creates it's own set of issues.
If you choose to build a sail that id more like a MH Genoa, smaller midgirth. You take the penalty of having this measured in as your largest jib/genoa. Your J is extend to the point that you are flying it from, the luff length increases etc. You end up taking a massive penalty for a sail you use maybe 10% of the time. AC 45/72, IMOCA, and VO70s all have tweaked rules to allow these types of sails. Most measurement rules lag behind new ideas and technology and struggle to rate them. IE the quad sail.