You're not that likely to see it unless you are a target of it (or you help run an instance), because most of it occurs in DMs or as replies to specific people. It's commonplace for different groups of people to have extremely different experiences on Mastodon, and for those experiences to be largely invisible to others, just because of the way the core systems work.
So absolutely believe people who tell you they've been subjected to racist harassment, even if you have never seen that.