I do, for example. I'm building apps that require mobile versions as well, and I don't want to build the same app three times, or even twice. Yes, React Native exists, but there's not as much codesharing between React web and RN as one would expect. Yes, React Native Web exists, but the whole experience is that of using a patchwork of open source libraries, some not as well maintained as others, to get the same experience as Flutter gives, first class.

Flutter on the other hand, for all three targets, it Just Works™.