Slave labor doesn't help much for high-tech projects. Slaves are mostly useful for manual labor that doesn't require literacy or complex skills.

Remember, before we invented tractors, if you wanted to move a thousand tons of rock a mile, it had to be done largely by hand. That is a use case for slave labor. Before we invented harvesters, cotton had to be picked by hand. That is a use case for slave labor. Domestic servants are a use case. But the invention of powered machinery made most slave jobs obsolete.