From my (limited) experience in (not even close to F1-level) racing, the physical sensations of the tires being near the limits of grip are fairly obvious. The reason why traction control helps so much is that a computer can adjust power accordingly vastly faster than a human ever could.
The drivers are extraordinarily focused professionals during races. I can't imagine adding some signaling system that would let the driver react any faster than he already was.
As a person that worked on a traction control system, a driver is not going to notice the difference between 5% slip and 15% slip, but 5% slip is much faster, (and faster than 0% slip) A light that went off at 1% slip would be an enormous advantage. It wouldn't be as good as a traction control system that is making 1000 changes per second, but it would be huge compared to nothing at all.
Not only that, but the traction control system can control wheel slip at a per-wheel basis. Something the driver can't do, with his single brake pedal.
The drivers are extraordinarily focused professionals during races. I can't imagine adding some signaling system that would let the driver react any faster than he already was.