You're thinking about this from the perspective of a developer, rather than the population at large. Suggest building a spreadsheet to one of your non-technical friends or family for something like this and you'll get a blank stare.
Using basic spreadsheet software is at least as ubiquitous as owning a home PC in the US. You can assume pretty much everyone can work their way through it, maybe with some help.
Meanwhile, you could easily argue that filling in the same data in a web form via a browser would be less intuitive for many people.
And regardless of which is easier, keeping the data private is clearly a priority.
Analogy: with a little time and some equipment, practically anyone could learn to DIY their own oil changes, yet very few do.
Also the top expense tracking app by number of users appears to be Mint (which claims somewhere near only 20MM users), which involves a great deal of data sharing and privacy questions. If we require a solution that keeps expense data private, it would shift a lot of favor to spreadsheets.
So many people use spreadsheets today. What are the non-spreadsheet budgeting or expense tracking apps that are highly used?