To vastly oversimplify how genetics and embryonic development works, the “default” for humans is female and men have genetic codes that overwrites these defaults during fetal development. To put it more bluntly, males have extra code which females lack.
That could explain why sperm+sperm could theoretically work using a stem cell as a mediator to remove the excess code, while egg+egg can’t do the same because you’d need to manually scribe the extra genes directly.
“Give up hope now before the pain of existence rips it away. Oh, also eat healthy, and do some light exercise daily that stuff is important too.”