well while not having factories on ships is unrealistic, gameplay wise it makes not a lot of sense either.
If a player/faction wants to have a planetary base or a station, he can make a factory. If they want to have a ship as their base, there's no reason to force them to have a planetary outpost or station for the sole purpose of crafting. You should always be able to have a factory, whatever your playing style is.
Now it dosen't makes planets/stations useless, because of faction homebase mechanics, and because they can be somewhat easier to defend (always easier to put thosands of shields blocks and turrets on a station than on a ship, because mobility is not a concern. And also, you can save all energy for shields and weapons instead of engines). And planets can always be used for mining, so you may want a mining outpost there (unless you're using a oversized slavager to remove it all at once).
So the idea is just that factories should not force you to play a specific way, and having just a ship as a player, or using a ship as a faction base should still allow you to do whatever you want, and be a viable playstyle.
And anyway, outside of the gameplay part, you have tons of roleplay or style reasons to build stations or on planets. and a lot of servers take them very seriously.
Yes, there is servers where players are mostly concerned by PvP and faction combat efficiency, but there, evryone is flying in doomcubes and either not very concerned by factories, or they'll just find ways of making them so that they can carry them around (like placing temporary and easily build and slavaged factories on nearby planets and stations when they are needed), so we shouldn't care about these guys.