Purely a guess, but I doubt that they perceive a real market for such a product. Most people who use shotgun microphones professionally, don't regard them as interchangeable with other patterns--either in the way they function or the way you use them. If the user wanted to switch capsules in the field, s/he would probably have to readjust the whole boom mike setup, possibly with a different windscreen and shock mount--not just replace the capsule.
Applications for a shotgun capsule on a Colette extension cable, tube or gooseneck don't come readily to mind--plus professional film and video shoots these days tend to be high-RFI situations. So again that skews things toward a complete microphone, not a component approach.
The other replies in this thread seem right to me as well. Some kind of low-frequency cutoff is really mandatory when recording dialog and most effects, and if you can build that directly into the amplifier (whether you make it switchable or not), so much the better.
--best regards