The key is that while many fonts have representations for Unicode math points, math symbols and math display are their own things, and typesetting them is not t

The key is that while many fonts have representations for Unicode math points, math symbols and math display are their own things, and typesetting them is not the same as typesetting text. As with musical notation, math typesetting has its own requirements and rules. Knuth created TeX as a way to typeset complex mathematical formulae. TeX has math mode, a completely separate environment. There's more to typesetting it than choosing a font that has the desired symbols.