Here's a troubling sign for Wall Street banks: Companies are increasingly eschewing the advice of bankers on mergers and acquisitions.

Global announced deal volume stands at $2.62 trillion this year, its highest level since 2008, according to data provider Dealogic. Yet the percentage of that volume that involved at least one financial adviser slipped to 73%, its lowest level since Dealogic began tracking it in 1995.

The...