It's more than that. Female gametes are produced during development and are extremely limited in number. Ticking clock metaphor, etc. Male gametes are produced almost the entire adult life repeatedly. This is more important than the general public seem to realise because it's the main source of genetic variation due to sheer numbers involved.

Generating countless sperm means each single genetic packet may be genetically unique. Sure you can have siblings that are extremely similar in phenotype, I knew 2 brothers that were 3 years apart where the younger brother was the spitting image of the elder, but they will still be genetically unique and that's more likely because of the genetic material from the father than the mother.

Identical siblings is a result of something happening on the mother's end post fertilisation when a zygote splits into distinct separation embryos rather than simply dividing properly during development and doesn't serve to increase genetic diversity for obvious reasons.

The touted bone marrow donor method, or any other, for the creation of artificial zygotes won't have anywhere near the production capability of those found naturally and are still just as likely to cause genetic stagnation because all that does is take 2 existing genetic sources and combine them. Recombination during meiosis produces new genetic material which is the entire point behind sexual reproduction over asexual ala Red Queen theory. You don't want genetic copies over and over because the risk to diseases and other genetic components skyrockets as monocultures are piss easy to affect for viruses, bacteria, and whatever else.

tl;dr artificial genetic production doesn't exist. Proposed methods merely retain existing genetic templates and don't hybridise them anywhere near enough to create new ones which are important to avoid genetic stagnation and collapse from diseases.