1. There are significant barriers to entry in search that do not exist in the browser market, including learning effects, economies of scale, and network effects. Any enhancements to the technology (e.g., algo) or user experience (e.g., UI) by a competitor must provide substantially more value to searchers and/or advertisers to overcome the significant network value (effects) in this market.
2. Despite Google's size it's still quickly responding to market and technology changes making it difficult for a competitor to chip away at its market share. For instance, despite significant investments by Microsoft (and some creative and smart decision-making), Bing has only been able to gain incremental market share, primarily at the expense of Yahoo. Google, thus far, has been able to sustain its dominance, commanding about 90% of the global search market.
"While Bing hit 4.37 percent and Yahoo dropped to 3.93 percent, Google still dominates worldwide with 89.94 percent of the search engine market share, the web analytics company reported."(March 1, 2011; http://blog.searchenginewatch.com)