As a main mage, the fact you think we should distribute magic pen on multiple item tell me that you don't realize how broken it will be to have multiple option like lethality do. And also how pointless it would be to have 4/5 pen mag by item, it would hit some champion too hard.
Diversity is a lie, there never had been any diversity for mages except "Morello or Graal" which was a fake flipcoin.
You always built the same item.
Ever.
If you didn't, it mean that your champion follow an other SPECIFIC set of item like rushing liandry or leachbane or whatever.
Also there is mages that use hextech probelt or zhonya first, etc. Like lissandra or ekko or vlad.
Problem is that luden is too good of a generic choice when seraphin is for champ who spam spell (and was broken when it wasn't cattering to that), and the other item is too 'weak' compared to the two but its big ass active could actually be far too strong if it was buffed improperly.
I do agree to improve some AP on some item to make them more attractive but let me you this:
Mage will ALWAYS want burst dmg and whatever make them scale fast.
Old void staff, DFG, luden, old morello, etc.