Maybe you're wrong, a two-part answer.

First, rich westerners have kids late, and EA is young - people having a child in their mid-to-late 30s isn't uncommon.

Second, the EAs I know with kids don't necessarily talk about them, especially in EA circles. so your sample that implies there are very few EAs with kids is probably skewed. (Edit to add: 6% of SSC survey respondents have more than 2 kids. Another 10% have 2, and a further 7.5% have 1. And the average age of respondents is 33 - per the survey, more than half of people over 40 have a kid, and 10% of those in their 30s do.)

 

Having kids is very expensive.

I'll give my reasons, but I'm not sure how well they generalize to the community.

I just don't want kids and never have. I value my time to myself a lot and the flexibility of having fewer commitments, and children would inevitably intrude on that. I'm also prioritizing my career, and I anticipate having to make career sacrifices for children, e.g. not taking a job in a new city (this weighs on my mind more than the costs of raising a child, but I'm not sure it should).

Also, raising an infant sounds pretty horrible.

Higher sensitivity and standards. People in these communities take life, responsibility, long-term planning, and suffering more seriously than usual. Some are waiting longer for the sake of being a better parent when they do have kids. It's emotionally intense, living in constant empathic contact with another person who has full depth of feeling and perception, but who starts out with little skill at reasoning, shielding, or emotional self-regulation. I'm told one doesn't fully grasp the seriousness and scope of the project until after the kids have arrived, and I suspect the non-parents in this community have a better guess than the average non-parent about how consequential, meaningful, and intensive the raising of another person will feel. That's quite a commitment! 

Slightly (only slightly) tongue in cheek answer:

Rational people are less likely to have children because having children is often quite irrational. It has been suggested that people are succumbing to a focusing illusion when they think that having children will make them happy, in that they focus on the good things without giving much thought to the bad. Rationalists are less likely to succumb to these illusions. (Note: this is obviously not true of everyone as some people clearly derive significant happiness from having children).

Not at all tongue in cheek answer:

EAs want to do the most good in the world and it seems that having children is unlikely to help with this, and in fact may make this more difficult. Children are a big responsibility and cost money, reducing time and energy for other pursuits, of which doing the most good will be one.

EAs may also not feel the need to have children as they are already deriving significant meaning from doing the most good. People who aren't (as) motivated by making the world better are more likely to need to do things for themselves to derive meaning.

There's been some interesting research done at Georgetown University that shows higher amygdala activity in the brains of extraordinarily altruistic individuals (in this case, anonymous kidney donors) compared to a control group in response to other people's fear (no similar increase in activity in response to pain or anger). Video for reference attached.

One reason for the increased activity is that "fearful expressions appear vulnerable and infantile" which triggers in mammals (humans, primates, dogs, lions) an alloparental care response. Dr. Abigail Marsh claims that the route of altruism is "the capacity, the desire, the skill, to care for other people's babies." 

Additionally, not only is the amygdala responsive to fearful stimuli, it's also said to be the "entry point to the parental care system," with a dense population of receptors for oxytocin.

EA is a hub of well-educated (less educated women = more children) altruistic individuals that are concerned with morality, not least including the moral weight of having biological or nonbiological children of their own. Pair these reasons with the evidence now supporting altruistic tendencies correlating to heightened amygdala activity, it could be that EAs are satisfying parental urges by simply participating in the EA movement. This participation could be enough for many EAs to forego parenthood of their own. 

All that said, birth control is sometimes fallible. I am 34 weeks pregnant with my second child and have found parenting extremely challenging and rewarding :).