Why?

I once sold a program for large network optimization problem in Excel. The calculations of the sewer planning for an entire district, a few million homes in northern Italy. Excel was the best for the job, integrated into a CAD package.

Safety? Wrong planning could lead to some problems as seenm in poorly planned cities (e.g. Houston), but I guess they are largely ignorable. "Common Mistakes in Planning of Sewer Networks and STPs" https://www.springerprofessional.de/common-mistakes-in-plann...

My district was properly planned, Excel was the best. Would not recommend doing something similar by hand.

There's a lot of engineering that just isn't at that level of critical. If it is, then they do it in their CAD software. If it's safety related, there might be just one person in the department who handles that kind of stuff.

Now, I have a relative who is a retired nuclear power plant operator. They did things a bit more formally there. ;-)