Because they're clearly not monopolies in the usual sense, just maybe oligopolies (note the "mono" in monopoly: it means "one"). If I want search I can use Bing and get results that are usually good enough, sometimes even better than Google's. If I want a phone, I can choose between Apple and Android, and maybe even niche brands like Jolla. If I want to message somebody, I can use Slack, WhatsApp, Skype, WeChat, Line, Telegram, Signal... If I want to post pictures or videos of myself, there's YouTube, TikTok, Facebook/Instagram. If I want to buy ads, I can buy from Google, Bing, Facebook and Twitter. They're only monopolies if we define the class of businesses so narrowly that each business model is a new class, such that each firm is a monopoly.
Contrast that with e.g. home internet, where in many parts of the US there's literally just a single company providing internet connectivity, a single ISP, and it's illegal to start a competing ISP.
Contrast that with e.g. home internet, where in many parts of the US there's literally just a single company providing internet connectivity, a single ISP, and it's illegal to start a competing ISP.