Why would you? There are a tiny share of small screen size phones out there, and often you need a substantially different design for a small screen. Basically you are doubling the layout effort on your designers and app developers (or let's say 1.3 times modifier at least), and those designs have to be maintained as well. The alternative is to really have a "one size fits all" design, but this will likely mean making…
pride in worksmanship, perhaps...
because 1% is still quite a few users for whom the experience sucks...
or perhaps just plain old empathy...
because not every customer can just replace their device...
or... i dunno... just avoiding someone posting embarrassing screenshots of one's app looking like shit on hacker news?