Statistics (real statistics, as in the number crunching, not issue-drive “statistics” i.e. “there are lies, d### lies, and statistics”) is a branch of mathematics, as are calculus, set theory, geometry, etc. Now, if you mean the “lies, d### lies, and statistics” type of statistics, I have to concede the point...
Computer science was housed in the math dept for a while. They finally escaped and are free to design database indices using their own secret and unprovable methods. Statistical mechanics is in the physics dept and the math dept doesn’t want them since they have taken to overt waving of hands rather than pretending to objectively prove everything. Bookkeeping uses numbers, too, but nobody ever thought of stashing them in the math dept or if they did it never got beyond thinking about it. Math is in a lot of things. Statistics has a proof of an exponential function integral that even in Calc III was said to be unprovable at that level; they are different people.