I think this originated from old arguments that say that the total _cumulative_ time spent reading code will be higher than the time spent writing it. But then people just warped it in their heads that it takes more time to read and understand code than it takes to write it in general, which is obviously false.

I think people want to believe this because it is a lot of effort to read and truly understand some pieces of code. They would just rather write the code themselves, so this is convenient to believe.