Because it is as accurate to claim that the Hamas Charter represents Hamas' unchangeable views as it is to claim 1 Samuel represents Jewish unchangeable views.
The Republican pollster Frank Luntz in his The Israel Project memo popularized the idea of promoting the Hamas Charter as the literal beliefs of Hamas today. If you read his memo you'll see he says that you shouldn't say things because they're true, but because they'll convince people. One of Luntz' students became an Israeli citizen and is now Israel's ambassador to the US.
Hamas has made repeated peace offers to Israel, and they've been repeatedly rejected. Ahmed Jabari was head of Hamas' military wing, had arranged the Giliad Shalat exchange, and was in charge of keeping the non-Hamas militant movements under control when Hamas was trying to keep a cease-fire with Israel. Jabari was working on a permanent peace agreement with Israel, and had just received the final draft, when the Israelis killed him with an air-to-surface missile in his car. That was no accident. The Israelis didn't want peace, because then they'd have to give up the settlements.
Quoting the Hamas Charter to prove that you can't make peace with Hamas is one of Luntz' strategies to avoid dealing with the facts. Luntz tells his clients that they should lie, and they do.