They do make towable systems. We operate six towable quarter-milers and usually move them twice a year. We'll start in the spring watering corn, milo, or something like that. When that's mature in late summer we move the sprinkler to the adjacent field and get ready to water wheat. In the spring when the wheat starts to turn, we'll move it back for the next year of spring crops. It's a royal pain, especially in dry years. Sometimes it gets hard to keep up with the water needs of two different crops.
I've seen drag lines once watering some new CRP grass, but here out west they're not very efficient. 100 degree temps, 30mph winds, and 9% humidity make for a lot of evaporation loss. Our goal is to apply the water as close to the ground as possible to minimize evaporation. Plus, drag lines take a lot of work and we switched to pivots because of lower labor requirements.
Pivots don't really take a lot of work aside from annual maintenance and the inevitable flat tire, bad gearbox, stuck tower, or some combination of the three.* We just drive by in the morning and evening and make sure it's still running and all the towers are in line, then check the irrigation motor. If you don't have any problems it just takes a few minutes a day per sprinkler.
*There are a hundred other things that can go wrong, but I think you get the idea.
We run six hydraulic drive sprinklers and two electrics. All but one of our towable units are hydraulic; we like them because they have on-board jacks that make turning tires a breeze. The electrics require us to carry a Hi-Lift jack through the field (or use the backhoe if we're lucky)**. The hydraulic drives seem to cause more problems as they age. This year we had a lot of problems with hoses blowing. It always seems to happen in the night, so by morning you have to work in a lake to fix it. We have less problems with electrics, but they're harder to fix when something goes wrong (heavy gearboxes, 480V power, etc.)
**My dad and I once turned all the tires on an 8-tower Valley in 38 minutes. We were using a Hi-Lift in ripening wheat, without a vehicle. That's got to be a record or something.
How much water applied per year is all dependent on the crop being grown, how much rainfall is received during the season, and the weather conditions during the year. There's been years here where guys started their wells in February and didn't turn them off until September. Those aren't good years.
Generally, corn will take more water than wheat or milo. Every year is a little bit different, though. We've got 6.5-7 inches of water applied to the wheat already. In a good year we'd only have to apply one or two inches to get the wheat up then it would be set for the winter, but droughts kinda throw a wrench into things.
Here's one of our T&L towables. You can see the jack hanging from the frame between the wheels.
Added the pic.