Changing/layering architecture adds risk. If you've got a standard way of working you can easily throw in on day one whose fundamentals then don't need to be changed for years, that's way lower risk, easier, faster

It is common for founding engineers to start with a preexisting way of working that they import from their previous more-scaled company, and that approach is refined and compounded over time

It does mean starting with more than is necessary at the start, but that doesn't mean it has to be particularly complex. It means you start with heaps of already-solved problems that you simply never have to deal with, allowing focus on the product goals and deep technical investments that need to be specific to the new company