I'm not sure that's a reasonable conclusion for most cases. Inflation is not uniform across the board, depending on who your employer is they may not have raised their prices by whatever the average inflation rate was, meaning the company as a whole is taking in less value (based on their prices hikes vs. the inflation rate). So the new value from the inflation rate is not "going" anywhere, it's just not there.

That's not to say you shouldn't argue for raises anyway, but just that it's not like every company is suddenly taking in X% more dollars because the average inflation rate is X%.

(A quick search seems to say that managers can't join most unions, but there might be some they can join. There seems to be no legal barrier, but rather that existing unions don't want them, and that managers seldom ask to join.)