It has an exceedingly dominant market share (FYI: Being a monopoly does not require "zero alternatives exist". Merely that those alternatives are at the mercy of the main one), which it uses to direct and control Internet protocols in the way it finds most beneficial to Google.
All new Internet protocols and 'standards' proposed by Google come with the implied threat that Google will do it anyways, since often Google implements it in Chrome before it is a standard, leaving everyone else to accept it or be rendered incompatible with Google, who has control of most of the Internet.
A significant number of websites now only test against Chrome and only support Chrome, leaving really only one option, often, when trying to interact with a given Internet entity.
And Chrome isn't just a monopoly, it's a monopoly they illegally use to manipulate the market and boost their other products. For instance, while competing search engines can bid for default placement on Firefox, Google gives it's own search engine exclusive rights to be default there, the monopoly search engine on the monopoly browser.
Recently, Google even introduced a method for banning competitors ads into Chrome, rendering Google ads the only "sure thing" for not having your ads arbitrarily blocked by Google.
Chrome is one of the clearest examples of why Google must be broken up. It's Ad/Search business cannot own platforms like Chrome and Android.