My buddy works at YouTube on the player and I asked him this same question. Basically he said there's resistance to changing too much on the interface in order to not mess with success. The good news is, they know the issue(s).
"resistance to changing too much on the interface in order to not mess with success" Oh my god. I see. Thx for the inside info.
Btw, all they have to do is change two lines of CSS.
.html5-scrubber-button {border: 0; width: 1px;}
But whatever, let's not risk anything here ;)