I have said these EXACT words, my good friend!

Short answer:

- Corporate inertia + familiarity + fear

Very long answer (rant warning):

- The XLS was used by multiple teams, from multiple sites, from multiple projects. It drove project-level decision making at the VP level. The person who wrote it was a genius, but there was no documentation or commenting, and over the decade after he left, it bloated Akira-style: many grubby hands had perverted it beyond its original use.

[Imagine if someone had written the most beautiful C++ & Boost (or C & GLib) numerical methods code, and then some boner noob came along and inserted their own bubblesort because they didn't understand Boost ... yeah, that kind of perversion.]

But because it was so important, and fed so many OTHER spreadsheets, it remains like a brain tumor pressing up against a spot so vital it could not be removed. I did a partial conversion to JavaScript and a MongoDB, but that was roundly shat upon because the main users weren't programmers and refused.

This is how very large companies work. (Most of the time.)