For one, the entry to compete is very low with something like Google Search, same with web browsers. I will admit that Operating Systems are winner take all monopolies and as any other such monopoly should be regulated.
But everyone has the freedom to download another browser, it's open protocols which anyone can implement. And search? You literally Ctrl+L and type bing.com from a user perspective. From a competitor's perspective you just write a web crawler and run it, it's a CompSci year 1 project.