Why can J1939 Parameters only have lengths of 1 bit, 2 bits, 4 bits, a Byte or 2 or 4 bytes and not any specific bit length?

Can anyone advise why Canbus parameters in Designer can only be 1,2,4 bits or 1, 2 4 bytes in length? I ask in that I have a device that Im connecting to that has parameters of 14 bits, followed by 5 bits then 9 bits lengths  etc because that optimizes their use of the bus for the data types they need. I can of course achieve the same by defining an interim Byte 1 through 8 integer math channel and use heaps of shift left and shift right and band functions to build my own structure.  and then add into byte by byte parameters.

If the parameter channel allowed length of any bit value then we wouldn't have to jump through hoops to build our own structure just let designer do as it does already for the existing options?

Or is there some other SAE standards related reason why that can't be done, in which case the manufacturer of the device Im trying to connect to  must be out of standards?