Plus the genetic situation for Athosians might be small enough that they actually need the extra sets of genes from the ovarian cultures. Yes, they can weed out defects a lot, apparently -- IIRC what Ethan said to the prospective father -- but there's only so far that can go. It's entirely possible that the status prior to EoA was the next best thing to cloning down the generations...

So shrinking their available DNA combinations might not have been genetically viable in the first place, and might yet be unviable even "today." Not to say that they might not offer it as an option in the future, once the eggshell technique percolates to them, but I suspect they'll be wanting ova for a good long time, simply because it's extra DNA.