If it’s bone dry with no sign of rain, everyone will be on dry tires regardless. No steward decision needed.
If it’s pouring rain with no sign of it letting up, everyone will be on wets regardless. No steward decision needed.
The decisions come in when there is a question on whether it is wet enough to put on rains, if it’s wet on part of the course and dry on other parts, or if the conditions will change during the race. Whatever choice is made is a compromise as the tire picked will be better for some conditions than others and nobody is really sure how those conditions will change over time.
Does the steward decide that it might rain so we all put on rain tires just to be safe even if it stays dry? Does the steward decide that we all go out on dries and if it does rain, whoever can drive their Zamboni the fastest wins? Does the steward decide that if it’s raining on half the course but dry on the other that we all chew up rains or struggle with dries? Everybody is guessing in these conditions. Why should everyone go by the guess of the steward what is best for everyone else?
Car racing will always involve an element of luck. Part of that is not knowing what your competitors will choose, and how your choices stack up compared to theirs. Wets or dries are just one of those choices.