Hamas has historical ties to the Muslim Brotherhood which Sisi and the Egyptian military consider terrorists, as well as a peace treaty and diplomatic relations with Israel. As others have said, Egypt is a military dictatorship with ties to the US and support from it, these make it complicated for them to just open the border (although I do think that Israel saying they wanted Gazans to flee to Egypt means they would not airstrike it should they open it in that direction but also logic isn't reliable in war all the time).