Not least of which: how does one make a platform with global dominance into a national public utility? How does one split it or manage it when it had to comply with so many local laws?
In addition, choice of pen has no effect on the ability to communicate your message while choice of social media does. Switching from Twitter to Mastodon isn't a solution because Twitter is the place where the public discussion is taking place.
Finally, writing instruments such as pens are something that anyone can easily make on their own. On the other hand, due to the required technical expertise, infrastructure, and network effects it is difficult to enter the social media market.