There are plenty of trail running races that make it work.

True, but I think trail running is fundamentally different.

The environmental calculus is different: it's just worse to litter on a trail than on a city street.

The practicalities are different: it's harder to find a place where a drinks table and a bin can both sit comfortably easily in range of the trail but 20 metres apart from each other. And it's harder for volunteers to collect the cups that missed the bin.

The goals of runners are different. It's much less normal to target a specific time in a trail run because the peculiarities of often less standard distances, different surfaces, and climbs/drops just make that less standard than road races.