https://voice.mozilla.org/en
“Common Voice is Mozilla's initiative to help teach machines how real people speak.”
The problem with Common Voice is the same resource problem plaguing open source efforts in general - for TTS data, it's not just the size that matters, it's also the quality.
For something like Sonantic, you need clean recordings from professional actors in proper recording environments (not to mention the in-house expertise to then filter these down to curate the training/test datasets). That costs money. A million people with laptop microphones will just never get there.
Sure, open source TTS seems to be lagging behind recent commercial offerings, but pharmaceutical research is actually an excellent example of a field with massive FOSS software usage. TTS is also "purely virtual" which makes it significantly different, and I would say significantly more approachable to open source collaboration.
We don’t see FOSS pharmaceutical research for instance, I believe for the same reason. The amount of coordination needed and the impossibility to separate TTS projects into sub-parts could also factors.