in typical Google being evil fashion, using their central authority on Android they now discontinued android sdk and made the sdk a piece of android studio. for no reason other than forcing everyone to beta test their "version 3" (ha) studio. so much so that you now have to download the whole studio, and pick the sdk from within it. there are still some ways to find the sdk directly, but they are going away.
disclosure: I work on the Android Studio team.
Why does flutter tells to download android studio, open it, run the sdk downloader, close it forever and move to another IDE?