Three thousand years ago, a lot of characters looked like what they mean, but over the course of three thousand years, some things are bound to change.
In those days, most people lived their whole lives without seeing any writing; many of them probably didn’t even know writing existed, just as most people today probably don’t know extremal graph theory exists: it’s not something you’re going to use often. Even the people who did write, wrote little. It just wasn’t important to society.
With the rise of Legalism, let’s say around 250 bce, writing became much more important, so writing materials improved, which led to the improved efficiency of writing. Writing became streamlined, less clunky, and therefore lost a lot of resemblance to the original shapes. If you know how to look, you can still see it in some words like 目 eye (sideways) or 井 well (looking at the frame from above).
aside: I used to have an American friend, S, who studied Chinese in New York. When people saw him reading Chinese, they’d ask how that was possible. He’d say, ‘It’s simple, Chinese characters are just pictures, so if you look at them real fast, it’s like watching a movie.’