The power to reorder is the power to profit

Maximal Extractable Value (MEV) is the profit that can be made by including, excluding, or reordering transactions within a block, power that validators (and the sophisticated 'searchers' who work with them) hold by default. Since transactions sit in a public queue (the mempool) before being confirmed, anyone watching can see a profitable trade coming and act on it first.

This isn't a bug in any single protocol, it's a structural consequence of public, ordered blockchains: whoever controls transaction order can extract value from that control, and MEV has grown into an entire specialized industry built around exactly that.