When you consider that it is impossible to reuse notebooks in each other... Or unit test them... Or that it is not especially easy to version control them in any sort of branch/merge workflow... Jupyter Notebooks are much closer to Excel spreadsheets than they are to what most people would consider actual programs.
>impossible to reuse notebooks in each other... Or unit test them...
It's quite easy to unit test notebooks. Whenever you write a function you want to test, write some unit tests (or even just asserts) in the same cell it's defined, then any time the cell is run to define the function, the tests are run too. Likely you'll be doing some manual tests when you write a block of code cum function, and it's not hard to copy the input crafted from those manual tests to a unit test.
Except Excel is in some ways much better designed -- it automatically recalculate everything if you change a cell, and makes it easy to link to other spreadsheets.
Spreadsheets have been insanely popular and useful. If the similarity is at all deep, there's opportunity in figuring out version control, unit testing, reusable packages etc, even if they seem hopelessly impossible on the surface.
