- Switching from a car to a van to a lorry is fairly low cost. You don't need to recreate your product (probably).
- You don't need to run distributed databases in a cluster to start.
But I think most importantly the decrease in dev speed and performance is an investment in future scalability. And I only imagine that this different will shrink over time to where for example a 1 node "distributed" DB isn't that different to work with than a 1 node "traditional" DB. And that small difference pays off because adding a second node doesn't have huge tradeoffs.
I agree that right now it doesn't make sense. If I was starting a new product I would still fire up a PostgreSQL instance. However I think that the day will come where the difference is small enough and software support is complete enough that we will start with something distributed, much like people don't often start with SQLite today, even though it is quicker to get going (also not a great comparison because it has less features).