As always, be wary of the word "just". Things that seem trivial can turn out to be surprisingly difficult. :-)
In this context, the issue is actually pretty simple: it's a factor of available personnel and time. At least in the US, most municipal fire departments don't have the budget for enough staffing to run with all of their companies fully staffed (by NFPA standards). I'd posit that 3-person companies are the most common configuration you find these days, with 4-person being rare, and 5-person (or more) being all but unheard of. Ironically some volunteer departments actually put apparatus out the door with 4, 5, or 6 personnel more often than most paid departments. The tradeoff is the staffing with volunteer departments tends to be highly variable, so that same department, on a different day, may roll a truck with 1 or 2 personnel, or not roll at all.
Anyway, back to the point. Let's a assume a typical "working fire": the first due company arrives, and there is a LOT of stuff that needs to get done, and in a hurry. Size up needs to be done, ideally with somebody doing a 360° walkaround. One or more attack lines need to be stretched and put into operation. A primary search needs to be conducted unless somebody on scene can absolutely confirm that everybody is out of the building. Vertical ventilation may be required. Additional lines may need to be pulled to protect exposures. A water supply needs to be established. A RIT (rapid intervention team) should ideally be in place to rescue any firefighters who become trapped.
Did I mention that you're starting out with 3 people on that first arriving apparatus (and the operator needs to stay with the apparatus as a general rule)? And then as the subsequent arriving apparatus come in, their crews usually slot in behind the first company, doing all of the things on the list above, which are of critical priority.
What isn't on the list of critical priorities (in relative terms) is "place cones around the supply line, or other hose that happened to fall in the street".
Now by the time the 7th or 8th due company arrives, you probably have staffing for things like putting out cones, doing traffic control, etc. But they're probably coming from the other side of town, or might be a mutual aid company from another department. So you could easily be looking at 10, 12, 15, or more minutes before they roll up. So it's during that first, let's say 15 minutes, of an incident, that you usually don't have cones put put, traffic control established (PD might be available to help with that, but they're dealing with their own staffing issues), and so on. And during this time is when things like "car running over the supply line" tend to happen.
It's unfortunate, but largely it's just the reality of the way things work today, based on the staffing that's available. If every company was showing up with 5 or 6 personnel, this stuff would happen a lot sooner.