The reason it exists is to prevent people from simply respeccing to fix their broken oath when trying to play one of the actual oaths.
P.s. The cost actually increases if you do it multiple times. It starts at 1000g then it goes up to 2000g then 10,000g where it remains for every subsequent oathbreak.
Of course, this is annoying af when you want to actually play Oathbreaker, since respecs are not only expensive but you have to rebreak your oath afterwards.
Which is why there are mods that exist to circumvent this mechanic.
