It is not just holes. I didn't want to analyze the entire problem in full, but FIY, this is how a concrete sleeper looks like:

https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.dayaen...

As you can see, the entire shape of the sleeper is adapted to the expected distance between rails.

In contrast, a wooden sleeper is just a straight log with no pre-made bumps for rails.