It was totally low hanging fruit. Try building a search engine today. The barriers to entry are much harder. Finding useful sites and filtering out spam are much more difficult now than when Google started. The signal to noise ratio on the internet has plummeted.
And that is just the difficulty of building an application. Actually gaining marketshare in search is a whole other problem to solve and also very difficult. Even if you manage to build a superior search engine you’ll have an uphill battle convincing people to use it.
This is what it means to have all the low hanging fruit cleaned out. Google built their application at a time when most people weren’t even on the internet and spam was barely getting started as a social problem. Sure, what they had was remarkable and innovative, but only because everything else at the time was so bad. But now? Different story.
You can actually read Page & Brin’s original paper online [1] and implement it yourself. It’s not very difficult. In fact, it was an assignment question in my 3rd year numerical methods class. Unfortunately if you just point the basic algorithm at a crawl (which you can download for free here [2]) you’re going to get useless results. The spammers are optimized to fool this algorithm (Pagerank) so you need to find a way to filter out the spam. And that is a very very deep rabbit hole!