Because they are not really decentralised. Vast amounts of cryptocurrencies as they are designed right now are disproportionately held by the early adopters, who will inevitably engage in rent seeking in one way or another (I wish I was wrong on this one).
Decentralisation has to happen through increasing trust between people, while cryptocurrencies are designed to replace trust. Yeah, current institutions controlling our monies are hardly worth the trust, but honestly bitcoin doesn't even remove that trust (it just shifts it to some random self-proclaimed experts at Tether, twitter gurus or god knows what else the crypto people are up to nowadays).
Decentralisation has to happen through increasing trust between people, while cryptocurrencies are designed to replace trust. Yeah, current institutions controlling our monies are hardly worth the trust, but honestly bitcoin doesn't even remove that trust (it just shifts it to some random self-proclaimed experts at Tether, twitter gurus or god knows what else the crypto people are up to nowadays).