Not all websites are built the same, some might require more than just five lines of CSS. Add to that QA to make sure it works. And purchasing the device(s) to

Not all websites are built the same, some might require more than just five lines of CSS. Add to that QA to make sure it works. And purchasing the device(s) to test that it really looks fine (plus setting up remote debugging if it needs adjustments) or writing something to replicate the notch behavior. It's one more thing to support in already many things we have to think about when doing web development (different browsers, OS, screen sizes, orientation, available features, ... and now the notch). Apple could have used the space around the notch for some browser UI for example and spared us the extra effort to make our websites look good (or even be functional) with that notch. At the end of the day, it's their customers that are benefiting from a good/better browsing experience.