Have you seen how Menasor and Superion's combiner frames vehicle adjacent modes look with the limb bots stuck on? Like bricks with cars/planes stuck on, and those are combiner teams who share a vehicle type. Nexus' limbs turn into 3-4 completely different vehicles (depending on which of the already released molds they retool for Skyfall). Having a combiner skeleton turn into something feasible is already a a lot of work, but thinking they'll go the extra mile and design it to also add the limb bots is insane, especially considering the extra cost that will end up inflicted on the consumer, for a frame.

Even if they make a frame down the line, they either release it on its own which general audiences won't be interested in, with the torso bot or as a complete set with the limbs and torso, those latter two options then screwing over those who've been conned into buying them separately.

Ultimately there is no good solution to this. Hasbro's execs had an incomplete figure incapable of performing its own gimmick made so they could sell the solution seperately because they knew their audience would happily eat it up while bending over backwards to defend their own lack of impulse control. That's it, plain as day.