I wouldn't consider the revenue streams of any of these companies to be meaningfully representative of potential market size in any way. Pretty much any niche market you can think of will be 100's of times larger. You can literally put pretty rocks you pick up in a streambed and generate higher revenues. With a little hard work and luck, some of the companies listed will grow 1000's of times in the future (but they'll still be considered "small businesses").
To argue the other side, an old boss of mine would say that some of the products listed are really features. One of his favorite mantras was: "A feature is not a product and a product is not a company."
To respond to the "is it really just this hard", well... yes and no. If you are unwilling to market/promote/sell your product or just give most of the value away, then sure, it's very very hard. However, if you're willing to hustle on the "business" side of things, then it may not be that hard. One thing that too many techies get wrong is that they try to be innovative in everything. Don't -- just innovate along one axis. For everything else, just copy best practices and you'll be fine.
Side note: commission driven sales is a viable strategy for some product categories in some markets. But for most, it's not. Remember, in business, almost everything is context dependent. Any good tactical advice of "do X" is also bad advice in a different context. And vice versa.