There’s only one reason- Sometimes human beings can do things that software can’t.

For example, by asking the right questions they can ferret out deductions that software might have missed. Or by getting to know your situation, they may help you formulate a strategy to minimise future taxes or answer other financial questions you might have.

While most modern software does ask you questions, provide answers and try to help with strategy, it will never be a match for an expert human brain.