system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] So why don't the humans just crank Space Marine production up to 10000000000% and crush every other race?

I think the reason was something to do with a hard limit on their numbers set by the Emperor, but there have been plenty of times when the powers that be have decided to disregard his orders, why not in this case?
gpt
Most of the gene seeds have become weaker or corrupted over the millennia. This is due to genetic mutation, Chaos influence, or radiation exposure. Several of the gene seeds had flaws from the beginning. Gene seeds need to be screened for flaws and culled if they are unsuitable. This is problematic because a Space Marine only produces two sets of gene seed in his lifetime (and if he falls in battle the second might not be recoverable), so the supply is extremely limited.

Furthermore, the recruitment process to turn a human into an Astartes does not have a 100% success rate. Some chapters are lucky enough to have a 1 in 10 success rate. Some have such a flawed gene seed that they can only manage 1 in 100. Some gene seeds are so failure-prone that their chapters are actually shrinking, each gene seed is producing less than 1 Astartes on average. Space Marines are created at a very slow rate, which limits their numbers greatly.

Finally, the last time the Emperor sanctioned an accelerated Space Marine program there were severe Chaos mutations. Corax had to personally give the Emperor's Peace to many of his own Astartes.