Also, it makes it easy to play a female DB without having to constantly justify why you're not pregnant.
I'm baffled by the idea that Terrestrials need to justify why they're not having kids
right fucking now in general, day-to-day life. Male or female.
I realize that if the goal is to optimize pregnancy for maximum DB kids then there would be a reason to push for constant pregnancy; however, that's not most people's goal in the setting. It's generally a mix between improving their standing, pushing some ideal, and doing what they enjoy. DB culture is long past the point where they want to maximize the number of children; that's supposed to be what thinned the blood so not all their kids Exalt (because they had kids with non-DBs).
Dragon-blooded women have decades more than mortal women do; where mortals might say "oh, in a few years", Terrestrials can say "in a decade or two, I'm focusing on my career". If someone tries to force the issue, then they have to force someone with super-human skills and superpowers to go along with what they want, which is a
terrible idea. In order to have anything resembling a reliable chance, you'd be tying up the time of 3-4 Terrestrials per pregnant woman (though I suppose if you put together a group home and force them all to stay there under guard you could cut it down to 1-2 Terrestrials each) instead of just waiting a while.
Getting into more mathematical reasons, a Terrestrial who lives 150 years and has a kid (on average) every 30 years will have a total of 5 kids. That's an entirely reasonable size for a family, and if each of the 5 kids has an average of 5 kids then (with 60-year generations because fudging) then the first couple, assuming they started about 50 years into the Scarlet Empress's reign (i.e. 700 years before canon start) would have a total of 48,828,125 kids.
So yeah. To get numbers resembling canon, 99.9% of those kids need to die. That's not a joke or exaggeration; 0.1% of 48,828,125 is about 50,000.
(If we account for early deaths by saying they each only average 3 kids who have 3 kids, etc., then you still get 177,147 kids as of canon start.)
You don't need to justify female DBs not being constantly pregnant because
exponential growth is nuts. Also, they have super-powers.