Probably, because several factors have conflated and reinforce each other:
1) Historically, everything on the web used to be "free" and this is still an implicit assumption for internet users.
2) Payments are not part of the HTTP protocol; micropayments never materialized
3) The web is too good for advertisers - targeting, reach, scalability are unmatched compared to off-line advertisement. This supports p.1 above and hinders p.2.
It's a vicious circle.
