That's a false dichotomy.

I can't speak for all web sites, but I reckon a combination of factors could explain why such a solution hasn't been deployed:

1. Advertising is ubiquitous, easy to integrate, and provides a safe revenue stream.

2. There is little to no infrastructure for the PPV model. Whoever builds it would need to maintain their own version of it.

3. People expect the web to be "free". This is even true within technical crowds who understand that it's really not free. And a large part of that group doesn't mind advertising.

So, really, it would require a substantial amount of effort to implement, it would add additional friction to users, and ultimately only a minority would appreciate it.

Had this model been in place from the beginning of the web, things might be different today. Alas, if my grandma had wheels...