I don't think anyone is handwaving. They are called the Living Gods, so presumably there are others, likely because they have taken the most active role and are approachable to actual mortals. That and they play the most significant part in the story being told.
There's plenty of indication that there are other gods, beliefs, and probably deities. From the Old Trees and Wyld to forest goddesses depicted in a stone fountain in the Frostwood itself, it even still has a spark of divinity in it and can manifest to clear corruption (and bang Champ). The Seven are just the main, well-known, and established deities. Elder Scrolls has the Nine Divines (or the Eight and One, depending on your view of Talos), They could have made the 104 Divines. But do we need the writers making 15 more deities just to make the entire world match an arbitrary number that no one could really agree on? How many do you want? 15, 37?
No, because they've laid the groundwork and have shown that there aren't just Seven religions (or one, assuming you consider worship of The Seven as one religion). If the story calls for a Dionysus analogue to be the God of Wine, I don't doubt they'll add a spirit, demigod, or minor deity to do it.
When it's needed they mention it. Atugia's clan worshipped spirits or followed their own paths for generations or even longer. Anubians are believed to have derived from their god Aurera.
I don't think the writers need to plot out thousands of possible entities; one that governs leaves and the one that watches over ducks and one who created turtles. If they do, they'll bring it in as the story calls for it. But I think I'd rather have them focusing on the story at play and in the area it's focused on, not making the entire planet of Savarra match Earth. There are other gods or beliefs and religions and the evidence of that is well-written and woven into the story without going off out of the 'play area'.
If the story was set in the Native American tribes of North America before 1400 AD, would you think that the only gods that existed in the whole world were natural spirits named Coyote and Thunderbird because not one tale or part of the game mentions Amaterasu or Hades?