Why is stacking needed

Hello guys

I have made some research about how the Stack consensus mechanismus works and I find it very interesting that is using the Bitcoin blockchain as a underlying consensus layer.
From what I understood, in the Stacks protocol miners commit Bitcoin in order to participate in Stacks mining, while the Stackers commit Stacks tokens and are then paid by the miners in Bitcoin.

But I don’t understand why the Stackers are actually needed? Would it not be just sufficient if there are the miners. Each miner would create a Bitcoin transaction and then there would be a random function that picks a winner among those transactions? In this scenario no Stackers would be required.

What am I missing?