Elon was surprised too. The model 3 was supposed to have a fully automated line. He couldn't make it happen. Industrial robots are bad at wiggling stuff into place, and hopeless at handling error cases. https://qz.com/1261214/how-exactly-tesla-shot-itself-in-the-...
Elon is the biggest day dreamer who imagines everything can be solved with technology around the corner in 6 months, but has a hard time acknowledging when the harsh realities hit him in the face, that real world engineering challenges aren't as simple as those in building successful websites.
He's gotten further with subsequent products. A lot of the learnings with model 3 went into the model y and later iterations of the model 3. And of course cybertruck.
Industrial robot usage and other manufacturing innovations are a big reason they are leading the industry in manufacturing cost and speed. Other manufacturers are starting to emulate a lot of the things they are doing with structural battery manufacturing, casting big parts, etc. The likes of VW and others are (by their own admission) behind and struggling with the number of parts, amount of manual work, etc. It might not be as automated as Elon Musk would like but it is pretty impressive.
It will be interesting to see what they do with their upcoming cheaper model. He seems to be hinting at the notion that further manufacturing improvements are going to be a big part of that.