Google Search is far more popular than the next runner up.

Imagine a front-runner in a marathon race who doesn't care about setting records or beating a personal best, but only winning the prize of that race. He or she looks over her shoulder and can barely see the next runner in the distance. That front-runner barely has motivation to maintain pace, let alone surge ahead.

It will take a runner-up who starts to catch up to Google in popularity and seriously eat into their search business for them to do anything.