Yappadappadoo's Whitepaper states the following:
There are only two reasons why we have not yet seen the rapid spread of PWA. Firstly, these high-performance, first-class applications are based on new technology which is just reaching ready-to-market status. Secondly, app-developers are only now catching up with the expertise necessary for producing PWA. Of course, for purely commercial reasons, it is not possible to find this type of application in the dominant app stores of Apple and Google: web apps will eat into their business!
There is sufficient amount of truthfulness as Yappadappadoo cites reasons as to why we have not seen much of PWAs that could battle Native Apps and beat them to the ground in terms of features and availability.
The lack of developers (not because no one has moved into the PWA ecosystem, but because the technology is quite new) is a major contributing factor of the limited PWAs we are seeing around.
And yes, the commercial & economic reasons behind the powerful app giants such as Apple and Google is relatively true, if one comes to think of it, as it might simply disrupt people's choice of native apps which they already have developers for.