How annoying, why won’t it let you undo it even after changing characters ?
Because your opponents get used to what perk you chose, being able to freely swap them up just makes learning what 1 then 2 of the perks you chose out of the 4 useless.
They can’t allow this because some perks are for ults. If they let you change you just use the non-ult perk until you have ult then switch for your ult. That would be broken.