Software startups are extremely lean companies in the beginning. That means the vast majority of the first few dozen employees are software developers, with maybe a UI guy, a couple PMs, and the leadership team. And almost all of the software developers are male, because the technical talent pool is extremely male and out of that pool, the people who are willing to work at an unproven startup is overwhelmingly male.
That means in the formative year(s) of the new startup, the company culture is created and nurtured by an overwhelmingly male workforce, which invariably means growing pains if the startup is successful and needs to start adding more non-technical roles like HR, marketing/sales, customer support, etc, where the talent pools for those positions have a more balanced gender ratio.
Even the first few female employees of the startup are much less likely to take the side of the new female employee because since they got in early, they’re usually higher up the food chain and have sizable equity stakes, which means they’ll put the company’s interests ahead of a new employees’.
This is how it’s always been. Microsoft was once a den of unchecked male nerd iniquity once upon a time.