In agriculture it's b/c those jobs don't steadily offer $10.50 an hour. Most ag work pays piecemeal, so you might get $.25 per pint of strawberries you pick or $.17 per tree you plant. During the height of harvest that might be a good deal but there are long tails on both ends of the harvest where you won't be making very much money.

Then, your harvest will end and you'll have to move to pick the next fruit. Some crops have longer harvests and some area have more types of crops that have staggered harvests. You'll see undocumented ag workers start in S. Cal in early spring picking berries and go up the coast ending in Washington and Oregon picking apples by the fall. In that time you've gone from blueberries, to strawberries, to raspberries, to cherries, back to black berries, to peaches and nectarines, to apples and pears. Each of those fruits would be a different farm and a different geographic area, sometimes multiple areas for the same fruit.

After apples you go back down to California for table grapes. Then wait out the winter until you start again or you can work for a forestry contractor planting trees until spring.

So to earn an average of $10.50 you would have to move at least 4 times from California's central valley to western Oregon and Washington and then into the eastern parts of those states before returning to California.

On top of that labor contractors will be taking parts of your wages for various services they provide, like meals or transportation to the fields.