Owens ran a 10.2 on cinders (I.e., ashes from burnt wood) starting from holes he dug out for his feet using spikes that were probably slower to run in than an everyday running shoe of today.
I couldn’t find it, but I’m pretty sure a study was done to compare how Bolt would have performed under similar circumstances, and his times were predicted to be much slower.
The 100m dash record hasn’t been going down because people have been getting faster since Owens’ time. It’s been dropping because of improvements in technology (I.e., running surfaces, shoes, clothing, starting blocks, etc.).