Oh you mean literally steal the source code?

Unfortunately I regret to inform you that this thing called IP Law exists and that definitely is 'stealing'.

Heck, even reverse engineering from stolen, but ultimately not directly used source code... generally also 'stealing'.

But uh yeah, Excel definitely does not have a publically available repo that can be forked.

Sure, you could I guess hack into MSFT and leak the source code, but if you live anywhere that has an extradition treaty with the US, you are going to jail forever.

... LibreOffice Calc provides a less risky alternative to this.

Like, sure, I generally agrer with your opinions, but our opinions are not the actual state of how the world works.