There's also the time factor. Something you build on your own will not have decades of development and polish behind it.

I agree that building your own is hard and something that can be done by exceptional engineers, but software is famously a winner-take-all industry, even if the optimal strategy for lower percentile programmers and median programmers to import generic modules, as long as building your own results in the best product (which it does, whether importing frontend or backend modules, if you hand code something, it's going to be the optimal strategy for building a winner product. Even programmers that are median or below average probably have an incentive to aim for being exceptional, because the best value of median and below average programmers is not building median or low value software, but having a shot at building exceptional software.

Also the challenge of building a generic module is much larger than building your own. A generic module needs to have flexibility for many different options and integrations, when you build your own you build just what you need and tightly integrate it with the product