Because conservativism is no longer a set of political beliefs. In the modern conservative movement (basically starting in the 80s, liberals and conservatives were much different before that) conservatives had social beliefs, like preserving cultural norms, promoting religion, and maintaining the nuclear family, as well as fiscal beliefs, like limited government, individual liberty, fiscal responsibility, free markets, and a whole lot of other bullshit that basically boiled down to, “we don’t want to pay taxes.”

Now, conservativism is really only about establishing an in-group and othering their opponents. Oppositions to trans rights may seem like an attempt to preserve cultural norms, but it’s real goal is to create outrage and panic over trans, “groomers.” Objections go CRT and DEI serve a similar role in othering people of color. “Wokeness,” is just a meaningless catch-all for, “enemies.” Similarly, fiscal policy is meaningless, and can be picked up and discarded whenever convenient; corporations can be deregulated and given tax breaks in service of the free market, but subsidized or bailed out whenever needed.

This is because modern conservativism isn’t a political ideology, it’s a fascist movement. I mean that literally, and while the meaning fascism is notoriously hard to pin down, I use Umberto Eco’s 14 properties of fascism. And, to bring this back around to your original question, fascists hate liberals because hating a group is very important to a fascist movement. The modern conservative hate for liberals is especially clear in Eco’s 4th, 5th, and 7th properties of fascism (disagreement as treason, fear of differences, and obsession with plots, respectively).

So, tl;dr: the one-sided hate that conservatives have for liberals is because conservativism is no longer a coherent political ideology, it’s a fascist movement.