Not sure if this is a rhetorical question or not. Pretty sure it’s excellent bait and you’ve just managed to goad me into doubling down on armchair psychologizing (well played!) but for anyone who is wondering, this is my take:

Liberals believe in notions of meritocracy and the just world hypothesis etc.

When they encounter a person who disagrees with liberalism, they take it as a personal challenge to their beliefs (especially when these beliefs are sorta latent in the sense that they go unacknowledged by the person.)

Not only do they see you as a threat to their ideology but this is amplified by the fact that you posit a different solution to the problems that you have identified in the (liberal) world.

So, by being on the left, to them you represent someone who thinks they understand liberalism better than they do and that you think that you’ve got all the answers.

I think this is where a sharp distinction exists between the cultural liberals - the ones who are born into it and don’t question it so being a liberal is their default position and it’s just the background radiation - and the philosophical liberals - someone who has arrived at the position that liberalism is the best political economy by their assessment. I find I can communicate much better with a philosophical liberal than a cultural liberal, even if I find myself disagreeing more with a philosophical liberal.

I’m going to do something really odious here and make a comparison to religion. Stick with me though.

I find that cultural liberal is a lot like an evangelical in their zeal and their presumption that their ideology is the correct one to the exclusion of all others. This evangelical attitude is what makes them antagonistic towards other ideological positions and it also has that persecution complex bundled with it - anyone who disagrees with me is punishing me personally and, simply by existing, your ideas are a threat to mine and so whatever I do or say to you is an act of self-defence. (Unfortunately you see this same crypto-liberalism in baby leftists who are of the anti-authoritarian flavour. In my experience, a lot of the seasoned anti-authoritarian leftists [at least the grass-touchers] tend not to share this attitude but it does vary by degree.)

The folks who are philosophical liberals, I find, feel a lot more like people who are of a non-proselytising religion like Judaism, Hinduism, and stuff like Quakers - they are pretty sure in their own beliefs and they know why they are what they are but they don’t have a conviction that their ideology has the monopoly on morality and so they tend to coexist much more easily with people of different faiths or of no faith. Or in this case, a philosophical liberal tends to be able to coexist with a radical with much less friction and emotional investment.

So basically, to TLDR the question, what it boils down to is their belief that by being a part of the non-default position you are positioning yourself as better than everyone who is in the default position since you are convinced that you are right and therefore, by extension, you are convinced that the majority is wrong. The conclusion: you think that you’re better than everyone else.

(The reason why I brought up the idea of religion and analysed the question from the perspective of Christianity is because I’m basically riffing on the idea of ressentiment applied to liberalism but that’s a big long discussion, especially if you aren’t familiar with the concept.)