Please Note: This is not the same question as "what is considered work on Shabbos" because I'm just looking for why we had Shabbos before electricity. And for the person who wrote "your understanding of the 39 Melachosmare very untrue" that may be right because I definitely don't know all of them but during Shabbos basically all (I said basically) of the Melachos still have a connection to electricity. Also, the rule that you can't do electricity on Shabbos was made before electricity was found so why is that if they didn't even have electricity then?
We live in an eletronically-heavy world, so growing up, little kids have to learn "don't turn on lights on Shabbos" before they learn "don't use flint and tinder (or rubbing sticks) to make fire on Shabbos." I think that's what's driving your question.
In a pre-industrial world, plowing, planting, harvesting, threshing, winnowing, weaving, making charcoal and the like were absolutely part of daily life for everyone. Stopping all of those on Shabbos was a major change.
I don't think it's a well-phrased question exactly b/c of what @ezra mentioned. But, it is a good question wondering "what's behind a melacha?" The concept of melacha is an extremely difficult concept for even devoutly religious Jews to comprehend. The melachot were based on activity done for the mishkan.
Unfortunately, most sources that I've seen translate melacha as "work". Thus, many people equate "work" with "effort" or "labor". So Jews and, moreso, Gentiles are absolutely puzzled as to why one may run around an athletic field (strenuous exercise is prohibited according to many for a separate reason that is not related to melacha) on Shabbat as that seems like "work" whereas one may not turn on or off a light that involves minimal effort.
So, melacha has nothing to do with electronics directly. Many of the Shabbat laws, I find, I do even if I don't understand them. I implicitly trust a rabbi's opinion, unless it's clearly obvious to me that he's wrong. (E.g. if a rav told me a product was kosher and I discover that it is not, of course I won't eat it.) The point is that I don't feel that I need to understand why I do things. It's nice if I can, of course. If you want to understand exactly what is a melacha and what isn't and why they became that way, there are many resources to help you or confuse you :-)