It is not illegal to carry large amounts of cash.
It is that the DEA can legally take it from you, upon which you are guilty until proven innocent. Even if they never charge you with a crime related to the cash, you have to prove you obtained it legally or they keep it, legally.
It is literally highway robbery by the DEA, but the law allows them to perform this highway robbery. The law needs to be changed, but it won’t be. The police state is too powerful to turn back.
And some "conservatives" supported the creation of this police state because it was claimed to be necessary to fight drugs. Pathetic.