How exactly is Google.com different from a phone book in 1990? Phone books, to my knowledge, were not prohibited from advertising Southwestern Bell or Ameritech junk at the beginning, they were just incentivized not to do so because selling ads was more profitable.
I don't have a great answer for you, but I have another question: if Google isn't different than a phone book in 1990, why were phone books never a top 10 business in the United States?
Too hard to monetize, too high costs, competition by the phone company.
Everyone had one, they were ubiquitous. Without the internet ads are much harder to sell, huge barrier to entry for customer acquisition compared to google. Without real time auctions and targeting ads are worth less, i.e. you can't target search terms in a phone book, only prefixes, let alone things like demographics. The cost to distribute a book to everyone is a lot higher than the cost to serve some traffic. I suspect ad density was too low too.
