Where MEV actually comes from
Before a transaction is included in a block, it usually sits visibly in Ethereum's public mempool, a holding area of pending transactions that any node can see. That visibility is the root of MEV, maximal extractable value: anyone watching the mempool can see a profitable trade coming, like a large swap that will move a pool's price, and race to insert their own transactions around it. Specialized actors called searchers run bots that scan the mempool for these opportunities and construct bundles of transactions designed to profit from reordering, the classic example being a sandwich attack where a searcher buys right before a victim's swap and sells right after, pocketing the price impact the victim's own trade caused.
Searchers don't submit these bundles directly to validators anymore. They send them to specialized builders, who compete to assemble the most profitable possible block out of all the pending transactions and bundles they can see, then bid for the right to have their block proposed. This division of labor concentrates a lot of sophistication and infrastructure investment into block-building specifically, which is exactly what PBS was designed to make safe to concentrate.