I seem to remember Venmo and Cash App had near useless web portals. TikTok's web app is very poor. Reddit's mobile app has functions not available on web. I bet the McDonald's web site doesn't let you order for pickup and get the deals (does Starbucks?). CapCut's web site sucks, and their desktop app is missing a bunch of features the mobile app has. I'd guess an absolute ton of betting apps don't work on the web because they are trying to do good location checking. Does Shazam even have a web version? What about mobility apps like Uber/Lyft and the bike/scooter ones?
On the flip side of the coin, some places are locked to web apps because Google & Apple won't allow them to exist. e.g. OnlyFans and Playboy can't get in the app stores, but OnlyFans still manages to make several billion dollars a year, most of which is almost certainly mobile.
I think you're misunderstanding my conjecture. My point is that there is no technical reason these features can't live on the web. I'm not talking about the incidental or intentional decision by some company to force user behavior by not providing a web solution.
Yes, theoretically anyone could build anything. Building it is not, nor was it ever the hard part.
There’s no financial, political, or mass market incentive for browser APIs to have feature parity with mobile OS APIs. Approximately nobody wants to do what you’re asking for. If anything, there are incentives against doing this.