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If I'm to wager a guess, it's not really knowing how to properly approach the construction of such sets. If the sets are made to transform without disassembly, you'd easily reach the size and complexity of certain Technic sets for 16 and up, completely missing the main target group of the Transformers franchise.

That's not to say there haven't been attempts, all the way back in 2002 with Built to Rule! It's like Lego, the Transformers side had the main gimmick of converting "Trans Skeletons". That FLOPPED big time. Then came Kre-O which was more traditional with sets that had two main builds (Robot and Vehicle), plus its biggest draw, mini-figures or "Kreons". Later came standalone sets with a single build (think of them as expansions) and "Micro Changers", Kreons with two builds, figure and "vehicle", as odd as it sounds.

Built to Rule! and Kre-O on TFWiki.