Why offerings are not shareable through characters?

It won't ever change, unfortunately. The entire premise of taking your character to Prestige levels is the bloodied clothing; and in exchange, you lose all add-ons/items/offerings/perks for that character and get to re-level them. The reason for this is simply - it makes people have to play the game longer. If someone really wants the prestige sets, they'll have to bite the bullet and level 150 levels and go through perk RNG three times to do so. The idea is that once you have a character at P3 - that's when you worry about building a collection of items/add-ons/offerings and your perks.

It isn't intended to be able to play a single character and use everything you unlock on their webs (item/add-on/offering-wise) on other characters, because then you aren't putting the time into those characters and grinding out game after game to level those characters. The entire game is designed around an insane level of grind atop grind, resetting it, and grinding it all out again before you can finally say you have everything "done" on a character and move on to another.

The idea is that you will play games until you have burned through any items that you just absolutely want to use - or you'll get rid of them since they are just RNG and you can get them (aside from event items) again while you re-level. Or, you sit there at level 50 and never prestige the character and just go through bloodweb after bloodweb to replenish/build your inventory for that character.

At least the game doesn't force you to prestige once you've cleared level 50's blood web and doesn't just stop you from leveling that character up if you don't want to re-level them through prestige.