Look around you and note that there appear to be about three dimensions, and furthermore there's gravity. Also, quantum mechanics is true. String theory is the only theory that satisfies both of those conditions at once. Other approaches to quantum gravity aren't that far yet, but could conceivably yield those predictions at some point in the future. Today, though, there's only string theory.

You are still using “prediction” in a sense that I would say is not at all “risky”.

Put another way, so far string theory is an elaborate kind of curve-fitting — a mathematical construction that matched the empirical data available at its inception, but one that to this day has never predicted a novel measurement. Unencumbered by the theory, we would not have expected any recent experiment to produce different results.

Maps are literally curves fit to surveyed datapoints, and nobody questions their value. So what if string theory is nothing more than a map of physical laws? GR can't predict quantum behavior, the standard model doesn't have gravity, string theory does both and so is presently the only "theory of physics" in existence.

I thought I remember string theory positing more than three dimensions. Does string theory explain our perception of three at human length and time scales?

Yes, in string theory the extra dimensions are very small, leading to three macroscopic dimensions the same way the thinness of a piece of paper leads to two macroscopic dimensions.

Considering that it started with 11, or was it 26 dimensions, and had to go through some hoops to get it down to three, that statement is almost absurdly comical.