And this is a reason not to?
As a gaming mechanic, you could fix it in a minute, of course, by limiting the spell damage, so wizards faced a trade-off between certainty and extent of damage.
In world, however, the question is why they don't have as powerful a sonic spell, exactly because they have more foes it would work against. If monsters were created by wizards -- which would be an enormous world-building factor -- it might be natural to have them create them resisting the greatest chance of damage. Likewise, evolution might work in, if you gave it time enough to run, and there were a way that monsters could pick up resistance mutations -- though you might have to work out how to exclude player races, or how to include them. In either case, it would create a powerful incentive to turn to the sonic spells.
Perhaps it's just that it's easier to work with fire than sonic magic. And it's easier to have it arise spontaneously and naturally. Which starts to lead into questions of metaphysics and world-building that only oddball players would appreciate, and even in a novel would take some work to put into an amusing lecture.
And, to be sure, there are research issues. Many fictional works have characters making leaps and bounds in science and engineering because they do not realize that what looks obvious is only so in hindsight. There are plenty of good reasons to believe in the geocentric system, only starting with the way that we do not feel the motion of the earth as it hurtles through space. Likewise, in a RPGverse, sonic could be slowly building, over a matter of centuries, to equal fire in spellcraft. . . which does required the DM or novelist to contain character spell development.