Comment by Sharlin

The point is that you don’t have a "seek" operation available. You are given a bytestream and aren’t told if you’re at the start, in a valid position between code points, or in the middle of a code point. UTF-8’s self-synchronizing property means that by reading a single byte you immediately know if you’re in the middle of a code point, and that by reading and discarding at most two additional bytes you’re synchronized and can start/return decoding. That wouldn’t be possible if continuation bytes used all the bits for payload.