It's not even just that Excel is Turing-complete. It's that Excel is widely used and is its own programming language with a comprehensive standard library, and I don't mean VBA - entering the = character in a cell opens up a massive library of functions which can be used in a fashion sort of similar to functional languages, since everything is nested functions.

There's branching. There's Boolean operations. There's variables (cells, which is ultimately the point of the entire exercise). There's even a switch construct. There's even really simple web requests! All of these are of course functions. The only thing that's missing is function definitions (which can be done with VBA, but I excluded that specifically for the sake of this exercise).

It's not particularly pretty, but is usable by almost anyone, has good documentation, and a huge community of like-minded power users. As you mentioned, Excel doesn't have any of the constructs which help developers for other languages to avoid mistakes. The simple syntax, users which are not necessarily aware of good practices, and a lack of methods to implement such practices produces a perfect storm of factors to introduce bugs.