Why aren't vehicle mounted generators grounded?
I assume that the "generator" is a 110 VAC or 230 VAC alternator.

The question is "why aren't ..." but the inverse "why should they be" is much the same.
Sharptooth asked "what's the worst case ..."

The "why not" question is probably answered by "In general use conditions there are dangerous situations which can arise and which cannot be allowed for or controlled but which are prevented by true-ground-referencing the alternator system. In Fire apparatus environments the overall situation can be managed in such a way that dangerous situations are prevented from occurring, making grounding unnecessary. 

As is shown below, at least one potentially dangerous problem can be avoided by true-grounding the alternator output. This can arise when portable equipment is operated from the alternator. If there is a single load and it is vehicle located and power outlets to other equipment are not provided then dangerous situations will probably not occur.
Even with external power access - IF the equipment is rigorously designed and tested the the risk of problems is small.  If eternal access circuits are provided, if each one has its own isolation transformer or ground fault interrupter then risk can be adequately managed. 



General situation:

One very bad scenario can occur.
Consider a PTO driven AC alternator producing say 110 VAC and powering several items of equipment. Imagine that the AC system floats relative to true ground but is referenced to the vehicle chassis and that the vehicle is not formally grounded and sits on rubber tyres and is insulated from ground.  

If all equipment is in good condition, all is well.
If there is only on load and it is contained on the vehicle not too much may go wrong due to the earth if fault conditions occur. However, if "appliances" (such as lighting or tools) are operated on extension leads from the system a significant problem can occur. 

Imagine that a fault to true ground occurs from a phase / live connection in connected equipment. This could be caused by a fault in a 2 wire connected lamp or power tool. Or due to miswiring of a device (as happens) such that live/phase appears on device earth/body and earth/neutral  appears where phase/live should be. Place such a faulty device in true-ground - especially if fire-engine wetted, and you now have AC phase at ground potential  and the vehicle ground will assume  a potential of 110 VAC relative to true ground.
This would be a shocking thing! [ :-) ].
Or could be.