why don’t curly brackets after the for loop allow me to find factorials in c?

I have used rextester c compiler to analyze the run time of your program and I could see,

without curly brackets running time : 0.18 sec and with curly brackets running time : 0.16 sec, which might vary time to time for approx +/- 0.05 sec. And if you are using or not using a curly brackets for the single line code in a loop which I guess should not affect the run time of the program.

You can use some other compiler and try running your code.