In truth, only a tiny fraction of InDesign users use formulas in their layouts. But if you want to request it, you should post your request it on the InDesign UserVoice website where product managers will see it, and people who also support it can vote:
I know a lot of designers who prefer to create their invoices in InDesign where this would simplify things dramatically. Not to mention pricing and sizing tables or anything where formulas would be useful.
Thanks for this advice! I went ahead and posted it in the provided link.
I think the thing is ... some of such functions could be pretty handy. Adding up columns. Copying value A to cell B. Multiply each value in column 3 by the value entered in cell C.
But there is no end to what additional "missing" functions would then get asked for, and we'd end up with a slow, huge, cumbersome clone of Excel.