With a spreadsheet the only thing I have to do is download it. It is that simple. No registration needed, which is one of the main complaints I see here in HN. I really like the simplicity of just let me download the thing. I haven't used it yet, though...
A web app would immediately make me not interested in it.
The great thing about an excel file is you can share it with your co-founder/accountant/CFO without needing to create accounts, troubleshoot "I can't log in" etc.
How many people write a web app to do their financial forecasting vs plugging it into Excel. Say what you will about Excel, but the world runs on it.