Bc it's not. If mining "profitability" goes down, then the least profitabile miners drop off the network. This causes difficulty to decrease, so the existing miners have a higher chance to hit a block.

The profitability decrease is either caused by:
You're paying too much for power which will likely lead to more inovative electricity setups.
OR
Your chips are old ie new technological advancements are being made.

This has always been the name of the game. I don't see how that leads to centralization any more than most code being written is in English does.