Because the US has very unfree speech, despite what someone wrote on a piece of paper in 17-whatever year.

In general, the US ranks pretty low on most freedom metrics, except for the freedom to kill with a gun. In general, the more your country has to tell you you're free, the less free you actually are.

Many other countries explicitly do not have free speech in their constitution, but something more narrow, like freedom of opinion. In those countries, what rights the constitution says you get, and what rights you actually get, tend to be more closely in alignment.