Microsoft and particularly Apple used to make cutting edge (at the time) TTS available as a marketing gimmick, but then did not develop this further because TTS was only relevant for screen readers and people with impaired vision weren't really their primary customers. Then TTS made advances and companies started selling high quality voices at high price points. Now companies want to make as much money with it as they can. Improving "free" TTS for ordinary customers is not really a top priority of Microsoft and Apple. Moreover, since network speeds have increased, any improved end-consumer TTS will send the text to a server and the audio back, so that a company can collect and analyze all the texts and make money with this spy data. That's how Google's free TTS server works.