Answer:
The answer is "because the gravitational pull of other disk stars always pulls them toward the disk".
Explanation:
The disk stars bop up and down as they orbit the Galaxy because the gravity of other disk stars always pulls them toward the disk and not because of friction with the interstellar medium or even because of the halo stars keep knocking them back into the disk.
This disk is about 100 time wider than it is thick. This slight thickness is caused by the regular bobbing up and down of stars in the disk.
