Before I start: what Israel is doing in Gaza is criminal, the far right government of Israel is horrible, what the IDF is doing is disproportionate and needs to stop, and Palestinians deserve to have their own country.
I’ve yet to find an answer to this that doesn’t somehow relate to some form of antisemitism.
Politics.
Israel is a US ally and receives a lot of aid, which means criticising Israel is politically useful for Chinese, Russian, Iranian and allied propagandists and the useful idiots who uncritically spread their propaganda. It's no coincidence that the same people who often spend all day discussing Israel online, are far more likely to deny what's happened in Xinjiang, be opposed to western support for Ukraine, and/or ignore Russian involvement in Syria and Sudan.
Even more unfortunately, these useful idiots then uncritically parrot propaganda which is often very anti-semitic. For example, you'll often see people unironically claim that '''Zionists''' control all western media or even on occassion go on about how cunning the '''Zionist''' is. It's icky.
Unfortunately these are people who like to think of themselves as good, so they're unable to be self-critical. Vanity, it's the Devil's favourite sin. Few people admit their biases even if they're aware of them. It doesn't help that the Israeli right often labels any criticism of Israel as anti-semitic, which offers anti-semites and those who spread anti-semitic propaganda cover. "Zionists say any criticism of Israel is anti-semitic. I'm just being critical!"
Once again: this doesn't mean people shouldn't be critical of what Israel's doing in Gaza.
This comment is overly long, the debate is deeply polarised, people have come to their view on the matter through emotion so no rational argument will sway them, and I will convince exactly no one to moderate their behaviour or be more critical and/or nuanced.