Because it’s what Israeli politicians and government officials actually believe. They aren’t even quiet about it. It’s genuinely not hard to prove the genocidal intent of the Israeli government.
Unfortunately western media just tends to gloss over it all. I’m not sure if reporters can’t fathom the US supporting ethnic cleansing or if they just want to avoid the flack they’d receive by being honest. Either way, it’s unfortunate because well meaning liberals are left to assume Israel genuinely cares about stopping Hamas and aren’t using them as a pretext to ethnically cleanse the Gaza strip.