Dragoons have an extremely lengthy rotation with a lot of situational skills that can be used off the GCD.

We also have the lowest Magic Resistance (and most attacks that hit us are AOE magic attacks) so we have to be more careful in combat than other classes.
Also, poorly timed ablities such as timing Jump when an AOE attack pattern is on the ground also gets us killed.

As such, not only do we have complexity in doing our DPS, we also have complexity in staying alive. Many people frown upon Dragoons because they "die so much", but really we are just a difficult class to play that needs to know all enemy boss patterns so we know when to use our attacks.

A dragoon that is just pressing 1-2-3 is going to have such a substantially lower DPS that its clear they aren't doing enough damage. This also contributes to people thinking our DPS is low, because in order to do high damage as a Dragoon you need to be able to use not only your GCD rotation but your Off-GCD skills perfectly.