This might be a really stupid question, but why do fronts only form around low pressure systems?
Firstly, there are no stupid questions, unless it is legitimately stupid. Which this one is not, only those that are unasked in my opinion are.
If I understand your question right, you're referring to cyclogenesis and the fronts associated with them? I don't know enough about meteorology to for sure say they
only form on low pressure systems but that sounds right. From my understanding it's just the nature of the contiguous United States and the topography that influences the structure of the mesoscale systems that occur here. Like
every Nat Geo documentary talking about tornado formation, you've got a jet coming from over the Rockies in the west, as that evolves it produces a cyclone. With that cyclone you simultaneously have a Bermuda high (high pressure situated around the Bermuda region in the ocean) pushing moisture up through the gulf and into the low system causing warm air advection. As that low propagates east north eastward or really where ever it wants to go you've got the cold air from the north and west butting up against that warm southerly flow. Thereby creating fronts. From what I understand that's how it works.

If anyone is more knowledgeable please correct me if I got anything wrong or add to what I've said.

Hopefully that answers your question.
Cheers,
Quinton