In a combat situation, with other explosions and shit going on around you, it’s easier to feel two pops instead of three or whatever than to notice your bolt locked after three pops.

It’s designed for war, it’s not a toy for the range.

The rifle pictured is a FAMAS, and the bolt does not lock open after the last round.

Though you're not totally wrong, fewer bangs than you expect on the last trigger pull is pretty easy to notice.