Why not?

Add a fin to a baloon and you can steer it.

And once the baloon can hold direction in the wind with the help of the fin, you can add sail to actually get some propulsion in direction other than wind.

The balloon moves with the wind. You cannot steer it because from the point of view of the balloon it is not moving.

There is no keel on a balloon. Without a keel, one is only going downwind.

I think you should be able to on a wind current boundary

> from the point of view of the balloon it is not moving.

Mind == blown*

*Sincere, but pun intended

Wind is still flowing over the balloon and able to generate a steering force.

No, as Walter said, it's moving with the wind, i.e. it has the same speed as the wind. It's only if there's a sudden gust that you would see anything flowing over the balloon. I took part in a balloon experiment a couple of years back, essentially a weather balloon with several cameras, including one pointing upwards - we could watch (after we retrieved the cameras later) hours with footage of the balloon itself. There were various loose threads etc, and I assure you - there wasn't much movement! Occasionally there would be a little gust, when that happened the payload would swing around a little bit, but for the most part it was very quiet. Particularly at the highest altitudes.

This is interesting https://www.youtube.com/watch?v=jyQwgBAaBag

Forgot to include: We had microphones up there too! It was literally quiet.

The problem at 60k feet is actually you are above the weather and don't have a ton of wind to work with. Moving up and down in the minor wind currents is far more efficient energy wise and practically.