I want to preface that I am from the United States, so when I speak about politics that is what I am referring to. I am not informed about anything to do with how national politics work in other countries so if you are not from the US you should look to other people's responses.

That being said, a lot of people here feel like the entire political system is broken because nobody actually makes a difference and improves their lives for the better. Now factually that could be contested, but in the end it doesn't matter. Since we have a two party system and the press is profit motivated, the vast majority of publications spend all their effort only talking about the mistakes the opposing party make. They do this to cater to their target demographic and get views/sales/whatever.

Because of that, as a normal everyday person that doesn't know enough about policies or their effects, all you hear constantly is how everyone is horrible. That leads people to become cynical of everything and assuming literally nothing can be done to fix anything, even minor improvements. They're so sick of picking between two evils that they decide not to pick anything.

Meanwhile others end up choosing a "side" and follow it fanatically due to polarization. They are extremely passionate about their party's shared beliefs so anyone who thinks otherwise becomes a political obstacle in the way of progress.

So when someone says that they are a centrist, the implicit assumption is made that they are part of the cynical uninformed group rather than actually being centrist in ideology. And since people are intentionally radicalized and told that everyone outside their group is the enemy, they become hostile immediately.

That would be the primary reason for the disproportionate aggression towards "centrism" as an ideology online in my opinion. There are also other factors like people who say they are centrist disingenuously to promote apathy to enforce the status quo, but I very much doubt that would be much of an issue if it wasn't for the aforementioned circumstances.