Or it could be that dwarfs (and other sentient creatures) are actually super-infertile, and it needs a prolonged period of marriage-level proximity to get the wife to 'catch', of the kind that mere beasts seem to do without.
Even pandas?
Well, (RL) pandas are going extinct at least partially because they don't seem to be fecund enough. Traditionally, nature ensures that any animal that can't breed dies out. (There's human interference causing problems, which is one good reason why humans should be interfering to try and help them, as they are, rather than let nature weed them out.)
It is arguable that, like intelligent parenting has helped Homo Sapiens and its recent forebears to overcome the totally underdeveloped babies necessary to allow safe birth to occur, an intelligent civilisation may have co-developed matrimonial practices and breeding rituals to counter an infertility[1] developed by that civilisation.
Status-quo, in a way. At least within the scope of the usual breeding variances, with "large-litters, often" critters generally having a high infant (/pre-maturation) mortality rate[2] or non-breeding rate[3], compared with those that have single (or few multiple) offspring and invest a lot of parenting time (or preparation) into making sure that the next generation is spared the most immediate threats...
What we don't have, is an insight into what problems a pre-dwarfish population that retained a beast-like fertility might have encountered even while developing intelligence and culture and spreading across the world. Overcrowding and internecine warfare might be the obvious problems to arise, but it's hard to say, without extreme speculation, exactly how a more subdued breeding programme amongst a sub-species of offspring of the original proto-dwarfs would have given them advantages over a world of cousins also from the proto-dwarf stable but instead heading down non-dwarf lines of development that maintained high population production but that must have had some form of increase mortality[4]...
Ditto the other races, to greater or lesser extents. I can't even say if Kobold ancestry came from the same source as Dwarf/Elf/Human ones (or various other races, from raw or modded worldgens), as it's likely that some forms of racial development are 'universals' and could have been independently arrived at, as much as there being a single common ancestor tribe that possessed a parochial non-animal tendency which then merely diversified into the slightly different breeding frequencies and tactics employed by each of the traditional civilisations. Much like flight being developed independently by multiple lines of creature, and also eyes, so that while our eyes come all the way through a common ancestor with apes, chimps, and all mammals, it was an independent solution that popped up separately from the insects, and the crustaceans, but still shared with birds and reptiles and fish from the pre-amphibian pentadactyl fish-like ancestor (at least... Google or Wiki this thin for accurate details, I'm just going from memory and trying to compress it down). Anyway. Did I have a point with that? I think I drifted.
[1] Or, possibly, that an infertility crept in as a breeding disadvantage alongside a survival advantage (maybe that self-same rise in intelligence/culture), and either couldn't be bred out (because of it being closely tied to the advantage) or didn't (because it didn't have to be, with the happenstance rituals being a surrogate to the lost natural fecundity). Sociologically, the more fertile members may even have been beset by problems breeding (e.g. ostracism) in this co-evolved development of phenotype and societal mores.
[2] Such as in fish spawning, most of the young being fish-food themselves, quite early on.
[3] Meerkats and wolves and others having 'uncles' and 'aunts' of a normally non-breeding nature who largely defer to the alpha couple when it comes to perpetuation of the species.
[4] Hmmm... Speculation leads me to wonder if easy-breeding combined with high mutability, hardiness, longevity and far extended juvenile status to create those Horrors that are Clowns. Migrating beyond the caverns into the deeper depths, they are capable of spawning large numbers to fill the large spaces, but went for slower life-cycles so that what we see are the turbulent masses of juvenile delinquents, acting much as the Viking Hordes (largely consisting of second, third, etc, sons, who had no hope of inheriting their parents' estates... only first offsprings stayed to happily maintain the dynastic line, so it is said, while the others go off 'Viking' (to 'Vike', if you'll excuse the Anglicisation of whatever the original Scandiwegian root was)... ) whenever their domain is breached. It's a handy little tale about What Might Have Happened. And thus probably completely wrong.