A cell formula defines a computation, but not a function. For it to be a function you have to be able to reuse it (call it as a function) in multiple places.

For example, you can't define the abstraction "square" in Excel. You can compute 2 * 2, A1 * A1 and so on. But there's no way to define a construct which given x produces x * x, then reuse that construct everywhere you want to square things. Everywhere you want to square something, you must inline the computation. That's the absence of functional abstraction.