Disclaimer: I wondered the same, since 2014, and this is what I puzzled together for myself, read it with a skeptical mind!

I believe a lot of it can be traced back to the wealthy and to conservative think tanks / media control by right wing moguls.

Back in the 1960s and 1970s, conservatives were perceived as well-off business people trying to protect their own wealth (I’ve read that people used to say things like “I’m not rich enough to vote Republican” or children shouting “last one in the house is a dirty Republican”). You can even see old movies dunk on conservatives (i.e. take Stanley Kubrick’s “2001: A Space Odyssey” (1968) , at the beginning, with the satellite dish tower, the protagonist noses off about reactionaries being in control of congress, thus leading the country towards war).

This is the rather extreme election result from 1964:

Political map of the US in 1964

Because liberals mostly were Democratic Party voters, Republicans and their wealthy donors tried to alter public perception of liberals (i.e. make it undesirable for their indoctrinatees to be liberal). This included taking over the media (and Reagan conveniently cancelling the Fairness Doctrine in 1987, which gave political bias in the media some guard rails), then painting liberals as all things undesirable: arrogant, weak, clueless, leeches, etc.

Having a “hate object” worked so well that they kept capitalizing on it. Much of it was/is just slinging sh*t against the wall and looking what sticks, but think tanks are indeed looking at what sticks, so successful patterns get repeated. Some of these successful patterns I can see are installing a victim complex in conservatives (feeling their back against the wall, they lash out easier, ensuring anyone talking about conservatives is conditioned to use very soft gloves) and the two-year bogeyman, often trying to capture, redefine and vilify some prior existing concept (thus, when the campaign hits, indoctrinatees can find lots of “proof” online of this thing existing).

For example, social justice used to be universally agreed on as a good thing, woke used to mean remaining aware of systemic inequalities, now they make conservatives pop an artery. This has been going for a while (the “hate object” over time has been rock music, hippies, metal music, supposed satan worshippers, pen and paper games, paganism+atheism, video games, social justice activists, cancel culture, black lives matter, critical race theory, wokeness, …)

And I think, yes, your perception is spot on. This is, for example, what I get when I search for “anti-conservative t-shirts” (if it’s too tiny, try it yourself - they’re all anti-liberal):

Search result on DuckDuckGo for anti-conservative t-shirts, all results showing anti-liberal t-shirts

TL;DR: conservatives are intentionally made and kept angry. It keeps them unified against a bigger enemy (see Genghis Gambit), drives them to go vote and prevents voters from switching sides even if they do not like some things the conservatives are doing. Add to that Russia amplifying this division like there’s no tomorrow. They’re installing this hate for liberals both in tankies and in far-right bigots (and, as far as I can tell, anti-liberal sentiment is pushed into Russian society, too).