The same selection pressure that applies to every other disease. Symptoms make you less likely to spread. Fatalities make you much less likely to spread. Most of the diseases you carry are completely asymptomatic.

This is why island populations all get sick when a ship comes in.

Don't most covid deaths happen after you're no longer infectious? I thought a lot of that was from the cytokine storm.

But to the other commenter's point, if omicron DID get a more deadly variant that kills you after you've already infected other people due to presymptomatic spread, then why would that variant be likely to die out? It's already reproduced before killing the host

it's overall less likely to reproduce because we take measures against covid collectively. What spreads more unhindered, covid or the common cold?

A disease that spreads before it's symptomatic may not face negative selection pressure for some individuals, but on the aggregate people wear masks, avoid contact, even entered lockdowns, are more likely to obtain vaccination, and so on.

There's a stronger behavioral response to a more deadly covid than there is to one that is entirely harmless. It's not just death that reduces rates of transmission, and it's not just sick or symptomatic people who avoid contact.