I can't answer your question but like green hydrogen I'd guess there are practical reasons.. but my question is wouldn't that just contribute to greenhouse gasses?
In practice, I'd have reservations about widespread manufacture of methane. Existing methane infrastructure is often leaky and methane itself is a far more powerful greenhouse gas than carbon dioxide. That isn't to say that manufacturing methane shouldn't be considered, but it's extremely shortsighted to say that methane manufacture is the only storage technology that should be considered.