Chances are quite high you will never need scale. If you knew 100% sure you needed it when building, you would build for scale from day 1. But most/all startups and also for many projects in large corps, they don't know if they need scale. And startup statistics definitely show it is a very high chance you don't need it ever because you will be out of business long before that. It's mostly a waste of effort, time and money.
Is it? You trade retrofit costs later for maintenance costs and feature development friction every day until scale is needed.
You make a very good point.
Scalability is a two way street and those services don't scale small.
That makes business opportunities for more flexible engineers.