Lots and lots of people in Silicon Valley do that, and ultimately, that's what's causes market correction. If there are way more startups out there than talented engineers capable of building products, then the engineers can negotiate a much better deal for themselves. Or they don't and go out of business, but if that's the case, then your initial assumption that they have no real personal risk in the venture doesn't hold.
I know a senior engineer (Boston area, not Silicon Valley) that's made multiple millions multiple times as an early employee. She comes in to startups after they've fucked up their v1 so badly that they can't bring it to market, negotiates a very sweet equity package, fixes the product, and then cashes out when they IPO or get bought.