Not hamas, Gazans that have no hamas affiliation. Could they enter Egypt? Could they enter Israel? Are there any other neighboring countries? Could they get on a boat, or a plane?
While yes, some always stay behind, a great many flee. Most do not stick around. Mariupol was encircled quickly, look at the towns in Eastern Ukraine where they had a lot of warning before the enemy arrived. Jews usually fled, that's why Israel was founded in the first place, right? A place to flee to.
At any rate, in Gaza, almost nobody has fled. Some should flee, right? But they actually cannot. Physically cannot, prevented by other people.