The primary reason Java (and other programming languages) aren't, and won't be used instead of Excel is that Excel is interactive. About the closest you can get to that with a programming language is Lisp, but it's still half-way there compared to a spreadsheet, when you can see the data all the time as it is placed and updated.

The second big reason is that Excel just works, and all programming languages usually require installing tons of bullshit - editors, toolchains, whatever - to be able to work somewhat conveniently in them, and the moment any link in the bullshit chain crashes, you're SOL if you're not a developer. Yeah, we sometimes underestimate just how much minutiae knowledge we have that allows us to fix random problems in our tools without even thinking for a second about it.

So maybe Excel is unfit for purpose, but it's still the most fit for purpose tool available.