High school and college students would, like my friends and I did every friggin summer 20 years ago.
Now that labor pool is being forced aside in favor of illegals who will work for less and are paid in cash. Employers favor this arrangement because not only do they save $ in wages, they don't have to pay unemployment insurance; they don't have to pay bookeepers to do payroll taxes and they don't have to pay payroll taxes.
The simple fact of the matter is, due to the physical nature of thee work, these types of jobs have at most a 5 year cycle. That's why in the 'olden' days (pre-illegals), people between 18-25 were to be found toiling away outside in the hot sun. (Also why you could suck down a six-pack at the end of the day without gaining any weight.)
Illegals are not supermen - they also get burnt out and can no longer work the same jobs as they age. They themselves are trapped because younger illegals coming over the border displace them in the next cycle. But since they have no other marketable skills, don't know the language, etc, they can't move up the value chain. Instead, they become a drag on the economy as they consume increased social services.
What we have no is very similar to Dickensian England, where absent child labor laws, business owners just cycled through each generation of youngsters. Take a look at the political rationale behind banning child labor - business owners were foisting their broken employees on the backs of society, just as they are doing today with illegals.