If a startup company invests a bunch of money in sales/service infrastructure, and the product gets delayed, they are screwed. Lots of startups die because they spent money on sales/service infra too early and run out of money to deal with inevitable product delays. Obviously, you want the sales/service infrastructure to scale up exactly in line with the production ramp, but it's easier said than done. You can't trust the schedules you get from engineers because they are always too optimistic. There's less financial risk if you scale sales/service too late than if you scale it too early, so that's what startups do.
If Tesla had built out its service centers in advance to match its planned Model 3 ramp, the Model 3 ramp delays would have put them out of business.
So that's why things are the way they are.