Apple Watch apps can't exist without the iPhone counterpart app. Even if they could no one is going to pay money for the limited functionality they offer. There may be a few people that would but the vast majority will not.
My point was that as a developer you're going to invest your time and energy in tvOS rather than watchOS because there is FAR FAR more potential to make a product people want and to make money.
Developers invest their time where they can make the most money which is why we see the largest investment in iPhone development, followed by iPad, then Apple TV, and lastly Apple Watch.
None of these change the fact that you made an incorrect statement --- for all practical purposes you CAN charge for an Apple Watch app. It is irrelevant that the app has to be bundled inside an iPhone app. It's no secret that an Apple Watch requires a companion iPhone, so this bundling is unimportant.
What IS true is that Apple does a lousy job of creating a somewhat separate "Apps for Apple Watch" store. There is a (messy and disorganized) list of such apps available if you look hard enough, but basically it's a godawful mess. On the other hand, THE ENTIRE FREAKING APPLE STORE is pretty a godawful mess, so it's not like Apple is treating the watch any differently...
The issue when investing as a developer is not "where do people spend the most time" or even "where are there the most customers"; it is "where do I have a comparative advantage". Selling one of a thousand similar apps on tvOS is a less interesting (and less lucrative) place to be than selling the one high quality login app, or sleep tracking app, on the Apple Watch. If I were still in the commercial software business, I'd be working on Apple Watch apps rather than TV apps simply because it's virgin territory rather than the very crowded space of tvOS. But doing that requires a certain confidence in being able to deal with a new development environment and a new set of problems, from bugs in the API to new ways of debugging, and that sort of thing is not for everyone.
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I am not complaining; I am explaining why the Apple Watch is not selling much. The product is not a problem, but the price is.
How do you know Apple Watch is "not selling much"? And what counts as "much"?
Apple sells more than a million a month (12 million in 2015). That's more than they sold of iPhone 1. It compares pretty well to 3 million Amazon Echo units sold. It's a hell of a lot more than have sold of either Pebbles or any Android Wear model (and probable, though I am not sure, of all Android Wear combined).