That is just the closest layer of security, there are plenty of outer layers you can't see.
The agents not in suits are out in the audience profiling people who look out of place. Not a tough assignment with these idiots you could probably pick them out be smell alone.
When they pick one out they stand next to them and wait for them to make their move. That,s why the perp never got close to Trump probably had an agent right next to him.
If it is like common shopping mall security, they have spotters whose only job is to spot, not to intercept.