I absolutely agree that this is 100% possible with HTML5, and the only we’re not seeing it happening on a massive scale is a because of a cultural blind spot - as evidence, most of the comments in this thread. The technologies we have now, like Phaser, are vastly superior for building games for the web than Flash ever was (I was a full time game developer for 10 years, and a full time HTML5 game developer now.) And I believe it’s actually easier to monetize these using Stripe than through an App Store. Your game lives on a URL, which is just click away, and paying for it is just one more click - this is about 10 seconds of work for a user. And, you control 100% of your business from production to distribution. There’s a huge pile of money lying on the table here for anyone who can see this.
An interesting article on the topic (not written by me): https://www.fortressofdoors.com/tag/instant-games/
2. The reason we don't see them anymore is because it's far easier to monetize them on mobile.