Many people and businesses have no choice but to use Windows. It's nearly impossible to buy a computer without Windows, especially if you're picky about configurations. Even if the choice exists (Apple), switching involves considerable costs: buying all new hardware, moving to different application software, retraining personel, dealing will legacy and proprietary file formats.
In contrast, moving to a different search engine is easy and free. There are no barriers to switch.
That's not what a monopoly is. By that definition, no company has ever been a monopoly.