The Silicon Valley startup ecosystem depends on venture capital. If VCs are putting in money, they want to see a return (and a big one, because of the expectation that nine out of ten companies, at a minimum, will crash and burn). And they want to see it ASAP, because that's how the time value of money works, and because companies are burning money like liquid hydrogen as they try to achieve lift-off. So there's an enormous pressure on founders to produce, which inevitably selects for people who are prepared to sleep under their desk for the chance of having a breakout company (and, not incidentally, making an enormous return for their VCs, who are presumably sleeping soundly in their nice Design Within Reach beds). The system doesn't select for women, for people who have families or lives outside of work, for thoughtful people. And by not selecting for them, it actively pushes them out, creating a culture that rejects and is intolerable for them [...]
But there's more to it. What makes a startup a startup isn't that it's new (we call that a 'small business'), it's that it grows rapidly, ideally exponentially. That pushes startups toward bits, not atoms (near-zero incremental cost), towards anything that leverages Metcalfe's Law, towards dark patterns of nonconsensual behaviour towards users (like strip-mining Contacts lists), towards eroding user privacy, to dumping everything users have created when the startup is acquihired, and towards falling back on invasive online advertising because having a viable business model was a distant second to growing a user base.
It's not so much that tech bros are bad in and of themselves, it's that they're a indicator species for an ecosystem. Like an algal bloom, their overabundance is a sign that the balance is amiss.