Like many, I grew up during the time when most fruit & veg was picked by hand, mostly by women and their kids along with students (that's the reason for the long summer holiday). As a teenage I could get work for a week or two, and then if there was nothing going, sign-on the dole and receive it within a week. That's all changed now, firstly because many farmers use gang-masters or agencies for their staff as they just don't have the time for all the paper-work, and secondly because you can't just switch in and out of work/benefits as easily as you could.
Relatively speaking, you could earn good money picking if you were prepared to put in the hours and effort, more than, say, retail work. The same holds true now from what I see, but's a lot harder then just turning up at a fruit farm at 7 am, signing in and getting out to work for the day, and getting paid cash at the end. I suspect most growers have to go via agencies etc in order to meet the daily quotas issued by wholesalers and supermarkets, rather than the older method whereby stuff went of to a market on a daily basis and sold at the going rate.