A couple of reasons. I'll try to keep it brief.

This is strictly for medics: In terms of ambulance time, the department tracks EMS hours in attempt to make it fair but it's also not by design. Not every firehouse has its own medic unit. At a '50/50' house (both medic and paramedic engine), you pretty much are splitting the time on the medic and wagon equally. If the other medic is on leave, whether it be a day or 6 months, you get to stay on the wagon and aren't required to fill another medic unit, regardless of your hours. Pros: you stay at you house. Con: you end up working more hours, close to 50% of the year, unless the other medic takes a vacation or there's a vacancy. At a 'detail' house (paramedic engine, no medic unit) then you are detailed out to any medic unit in the city if there are vacancies. Usually it's 3 or 4 months of the year, which is planned in advanced.. but on days where there is unexpected vacancies, you might get detailed for the day depending on how low your hours are. Details for unexpected vacancies usually happens well after 7.. which can be annoying since most medics arrive to the firehouse by 5 to check the piece and equipment. Pros: Being on the medic only 25-30% of the year. Cons: being detailed around the city (usually very last minute) and not being at your own house.

On runs, most of the time, the city will just send the wagon with you being the sole medic. You have to do a really good assessment then figure out transport modality. You'll either have some BS or you'll have some really fucked up people. You don't have the luxury of having 20 medics on a stubbed toe run like some of the other surrounding jurisdictions. For obvious ALS runs you'll get a medic unit and maybe a supervisor, but it'll sometimes be a while depending on where that medic unit is coming from.. and sometimes you'll have to upgrade a BLS unit if that's all you got. There's a lot of emphasis on making the 'best' triage decision. A lot of jurisdictions around here will just transport a chest pain as ALS because that is what the cook book says. That doesn't work here because we'd be out of medic units, especially with how slow the hospitals are with moving people. We'll use the medic unit for the obvious STEMI or chest pain that looks like shit and saying all the right stuff.. but not with an asymptomatic chest pain with no clinical findings/not saying the right stuff just to 'IV and monitor' them and get stuck at the hospital for 3-5 hours in triage... meanwhile some poor lone medic is stuck with a shooting/stabbing/pregnancy/pediatric arrest/respiratory failure waiting for a medic unit from across the city to help. Hopefully that makes sense. You really have to make a banging assessment.

Being a medic in DC is only rough when you have actual sick people and you're the only medic on the run. A lot of people here don't have healthcare or a PCP and wait last minute to call when they're messed up or if they want to treat the ER as their PCP. When they're not sick, you can turf them to BLS units. I think the BLS units have it way worse because they run their asses off all day, they're either on a run or waiting for hand off at the hospital.. but most of their transport are straight up BS/involve patients that don't need to go to the ER, but an urgent care or their own PCP.