>>53119243To give an actual answer, it just an old irrelevant tradition they never got rid of. They made starters have such high percentage of being in order to prevent people breeding their starters early and either trading them around or just making an entire team of them in the early game. But nowadays we have stuff like the internet and even surprise trades that just circumvents all of that so in present day there is literally no reason to just make the gender ratios on starters 50/50 like the VAST majority of other Pokemon.
TLDR: Game Freak is either too lazy or stubborn to change an old tradition that just doesn't need to exist anymore due to modern internet.