I am a landscaper and would absolutely not be able to do my job without a large truck. I agree that for most cases a truck is not needed, but some jobs simply require their use.
Well, just from today, I had to have two trucks just to haul debris away from a job site (which we were both fuller that I'd like). Also something to consider, is that a lot of our work trucks double as snow plows in the winter, which is something a van absolutely can't do.
(I’m not a landscaper, but they seem to pack in everything from hand tools to those petrol powered tiller & compacting tools)