I am not an attorney nor am I an automotive engineer but my guess would be the TPMS manufacturers opted not to make the sending units serviceable to limit the potential for failure and resultant liability problems. I don't know of any modern day automobile manufacturers that still offer, for example, brake master cylinder repair kits, all in the interest of limiting their (and their dealers) potential to being sued.

With a seven year service life most new car buyers will never need to replace the sending units and the very nature of where the sending units are located and the environment in which they operate (electronic devices subjected to high centrifugal forces, vibration, moisture and ever changing temperatures) I can see why they would be manufactured as a sealed unit and have no serviceable parts.