The difference is, whilst clerics are usually dedicated to one deity, each deity does on have it's own religion. They are all parts of the same religion.
Ok, but each deity has indeed its set of values, common rituals, common goals that especially their clerics strive to achieve, and duties of the clergy (once again, to avoid misunderstandings, I don't mean preaching to the massess). The worship of a given deity is drastically different from that of another one.
Exactly. It has been done before, and without stat blocks and crunch there is no point in republishing what has already been done with "5e" stencilled onto the cover. There is no-one in the world who cannot use Google if they are all that interested.
A lot of things have been done before as well. The lore part in MToF has been done before, but many people still sang its praises. Idk if I'd buy this book when I can find pdfs of Faiths&Avatars, Powers&Pantheons and Demihuman Deities on the guild, because the depth of the lore 5e offers will never get even remotely close to that. However, there are people new to D&D who would be happy to have such a book.