bash – SETUID not working in my laptop


Suppose i have two files

total 8
-r-sr-xr-x 1 root root 34 Oct  8 17:36 openroot.sh
-rwx------ 1 root root 38 Oct  8 17:34 root.txt

Content in root.txt :

Hi I am root file.

Content in openroot.sh :

#!/bin/bash

whoami
cat root.txt

And now i am regular user (for example : person1). I execute command below:

./openroot.sh

The output is below :

person1
cat: root.txt: Permission denied

which means i cannot open root.txt.
How does it happen?