So whose equity is not worthless? I guess there's no point in having ycombinator around anymore then since all equity is worthless.

All new equity is worthless. If you persuade someone to buy some it's a little less worthless and if you demonstrate value by generating revenue then it a little bit less worthless again but of say 1000 new companies, 50 may generate revenue and 5 may get investment and 1 might and that's a big might, give you a return on your money. Ultimately you need to pay the software engineer for the risk they take and you can do that in many ways but I'd say unless it's a technical founder who really is a founder with you - i.e, they said to you, hey I've got this great idea but I need help to realise it, then you need hand over money.

Also - Why are they not going to be co-founders if the risk profile is exactly the same?

Cofounders get much higher expectations so they get more equity. Basically the same thing, just less equity.

Yes and I'm curious what pool of workers don't need to get paid but would be open to gambling their time on the riskiest of bets

What else what they put their time on? side projects that for sure don't make any money?

Their own startups. Your startup, from their perspective, is just your side project.