How does someone get a campaign medal with no campaign stars?

The campaign stars are for participation For a particular campaign on the ICM, ACM and now the GWOT and GWSM. Contrary to popular misconception it works the same way with the NDSM as well.

One usually has to tell the personnel people which campaigns you participated in in order for the campaign stars to be applied to the appropriate document (for Army types the SRB/ORB).

It has nothing to do with how many trips you made to what country for those espousing that point.

Example 2-16 Afghan Campaign Medal AR600-8-22
....(4) One bronze service star will be worn on the suspension and service ribbon of the ACM for participation in each designated campaign (see para 6–8 for further information on the bronze service star). (The designated campaigns for Afghanistan are listed in app B.)

I participated in 3 campaigns for two deployments to Afghanistan during the qualifying period. Consolidation I, Consolidation II, and Transition I.

Being in country for 30 consecutive or 60 non consecutive days qualifies you for the medal. There was a push for people to put the right campaign stars on the medal. To the posed question how can people not have campaign stars: either didn't meet the qualification or they don't understand the rules for their particular campaign medal and they just don't.

A bronze service star has a different meaning depending on the award. For example a National defense service medal uses a star to represent a person who served over multiple times our country has been at war. A campaign star represents multiple campaigns or in the case of the ICM PH
phases not necessarily deployments. I served during the "surge" so I have two stars (phase 4&5).

It could be that the person(s) in question does not know they are to have bronze service star devices on their campaign ribbons.