Because every build draws a new sample of game simulations, and no two samples produce the same lineups.
Each lineup is the best possible lineup for the simulations it was built from. A new build will sample a different set of simulations for each of its lineups, so you’ll never get identical lineups when building over and over again.
That's by design. Each build surfaces a different set of ways the slate could play out.
It also means two people building at the same moment won't get the same lineups, despite the fact that the two builds might be strategically aligned and find some of the same stands and fades. This is what prevents all SaberSim users from playing identical lineups every slate.