Comment by IshKebab

It was never the only hard part, but it definitely was a hard part (at least in most cases; obviously there are some monopolies with relatively simple software - mostly where there are network effects like WhatsApp).

But give me the source code for something competitive with Solidworks, Jasper Gold, FL Studio, After Effects, etc. and I'm sure as hell making a business out of it!

Furthermore while good software may not guarantee business success, it is pretty much a requirement. I have seen many projects fail because the software turned out to be the hard part.