This cannot be stressed enough. I've outlived generations of finance teams at many startups, and I've seen firsthand the masterpieces/abominations left behind in Excel. Imagine a dozen sheets with ad-hoc queried data copy/pasted from System A/B/C/D into Excel, with formulas that feed formulas that feed formulas. Sometimes columns are inputs (seasonality adjustments for monthly forecasts), sometimes their outputs (modeled growth * last year * seasonality adjustment) and more often than not their right next to each other and maybe they have different cell background colors or a black separator line. Maybe.
And this is just finance. For many e-commerce businesses, planning is done in Excel with equal zeal.
It would take about 12 hours to calculate, and would error out before finishing about 30% of the time. It needed to be run once a day for something reasonably important.
I don't use Excel much these days, but I do point people to a video if they do plan on doing anything:
* [You suck at excel - Joel Spolsky](https://m.youtube.com/watch?v=0nbkaYsR94c)
I worked for a company that used an opaque excel spread sheet as a part of its accounting system - turns out there where bugs and we found a massive short fall one of the contributing factors in the collapse of the company.
They're often built by specialists in another dept who definitely wouldn't consider themselves programmers.
Doing it 'properly' would probably mean having to spec put the problem, get a budget, maybe wait a few months for someone to look at it. And the same thing every time the requirements change.
Excel is available today and they can get started solving their immediate problem straight away.
After it's been in use for a couple of years and shown value someone takes a look and sees the Lovecraftian horror it's become.