There are mild returns to scale in running large-scale mining operations and as a result mining power seems to actually be somewhat centralized under the control of a small number of players: https://digiconomist.net/cryptocurrency-decentralization/
Not to mention that "decentralization" is a technical property and not necessarily desirable in itself. Users might care about fairness, avoiding sanctions, purchasing illegal goods, etc, but these are only weakly connected to technical decentralization.