The libraries are there for avoiding repetition. It is useful on its own without other benefits like implicit/explicit boundaries, readability by humans etc. If all lego(base unit software) pieces were unique shaped it might fit your requirements more efficiently but the cost of building those unique shaped legos explodes with the size of the lego piece required.

LLMs compress the known shapes well and fit them to solve for a problem but they are still not good enough to build from scratch a large new lego piece which fits a full problem perfectly. And such a large lego piece might not be the most efficient solution either and might be difficult to prove so.

> the cost of building those unique shaped legos explodes with the size of the lego piece required.

The cost of plastic injection molding is almost entirely due to the cost of creating the steel mold. My argument is: with LLMs we are entering a world which makes it cost effective to build many more things from scratch, which is akin to going from building things out of pre-fabricated lego pieces to 3D printing (agreeably not cost effective for everything, but for many things).