Having kids takes a lot of time and money. If you work till 5 you need to pay for childcare, which isn't cheap at all and might even be akin to doubling your rent. Mom is also working so you can't just unload domestic work and childcare to someone else to worry about. If you are entry level chances are you need to get there before the boss and stay there after if you ever want to not be entry level, and work weekends during busy season, so even that 40hr week can go up to 50-60. If you are a woman asking for maternity leave in an entry level job could jeopardize your entire career trajectory. Even in superficially liberal SF, VC firms have been known to discriminate against women entrepreneurs just from the possibility that they might be pregnant one day. People can scarcely keep housing costs below 30% of their net income.

People are waiting until they have more money and a job that actually gives them time to raise a family. The real reason why people are putting off having kids is that it's not 1950 anymore.