It's because the kind of massive training sets that are required for accurate speach/text recognition are difficult and time-consuming to amass. It's just too difficult for hobbiests at the moment.
It's kind of the same reason there is no good open source solid modelling software (like Solidworks or NX). The problem has been "solved" for years, but actually doing it is too mammoth a task for open-source software.
Creating training data is a relatively simple task though (even though massive in scale), which can be done by untrained people in a crowd sourced fashion. Creating a modeling tool is both difficult, requiring experts _and_ time consuming.
FreeCAD is starting to get good, but the problem is it lacks core functionality that Solid works already has built in. Admittedly, the underlying python calls are exposed, and you can use them... if you know how to make the appropriate calls.
When they get good GUI hooked up to the calls, it will be in parity with the pay solutions.
It's kind of the same reason there is no good open source solid modelling software (like Solidworks or NX). The problem has been "solved" for years, but actually doing it is too mammoth a task for open-source software.