I will always point to mlk as a response to this question:

I have almost reached the regrettable conclusion that the Negro's great stumbling block in the stride toward freedom is not the White Citizens Councillor or the Ku Klux Klanner but the white moderate who is more devoted to order than to justice; who prefers a negative peace which is the absence of tension to a positive peace which is the presence of justice; who constantly says, "I agree with you in the goal you seek, but I can't agree with your methods of direct action"; who paternalistically feels that he can set the timetable for another man's freedom; who lives by the myth of time; and who constantly advises the Negro to wait until a "more convenient season." Shallow understanding from people of good will is more frustrating than absolute misunderstanding from people of ill will. Lukewarm acceptance is much more bewildering than outright rejection.

To quote Malcolm X:

The white liberal is the most dangerous thing in the entire wester hemisphere. He is the most deceitful, he's like a fox. And a fox is always more dangerous in the forest than the wolf. You can see the wolf coming, you know what he is up to. But the fox will fool you. He comes at you with his mouth shaped in such a way, that even though you see his teeth, you think he is smiling.

All their supposed progress and opposition to capital only reinforces and propels capitalism, alleviating the need for fascism just for a little longer (which arises for the ruling classes when the majority of the population grows disillusioned with their lies, be they conservative or "progressive"). In the end only legitimizing the underlying framework (capitalism), without ever threatening it.

tl;dr: scratch a liberal and a fascist bleeds

Liberalism is a big-tent ideology that services Capitalism. Leftists want Socialism, Liberals want Capitalism. This is the divide.

For me it is a frustration over that liberals would rather endanger the liberal democracy by working with the far right than collaborating with anything considered left. It is very obvious in Sweden. The swedish far right has declared that the liberal democratic project is a threat to their nationalist vision -- it's not just me as a leftist saying this, but liberals, as in the Liberal party, said this about them. Then came the last election, the Liberals sided with the far right. Its down to two liberal MPs and they could force a switch from the far right to just center lib politics. But no. They rather want prisons for children than work with a socdem and maybe suffer to have a cap on profits on charter schools.

As an outsider, I’m curious why there is such a focus on liberalism in leftist circles?

Liberal politicians in the US tend to be deeply connected with the industrial and media elites. So we get a recurring cast of candidates who are milquetoast on a slew of popular issues, while they're lauded as "The Most Leftist Politician To Ever Think About Running For Office" in headline after headline.

Leftists who run are regularly denigrated as unrealistic, unelectable, and disastrous for the domestic economy by the same industrial tycoons and media magnets who push unpopular candidates and their dismal policies. So we're in this constant state of tension during election season, with a candidate like Joe Biden who receives enormous stacks of cash and tons of DNC support fighting against an outsider like Sanders or AOC who divert time from expressing generally popular sentiments to argue over whether they're well-dressed enough to win over a rust belt used car salesman.

What’s the relationship between liberalism and leftism?

Liberals tend to campaign as leftists and govern as conservatives. So they initially attract a lot of leftist voters, and then end up having to argue that said voters shouldn't ask for anything from the party once the election is over. Leftists tend to live on the political outskirts, looking for anyone remotely attractive to rally around, only to get taken for a ride by con-men in the liberal party until they finally burn out and stop engaging with electoral politics.

Because the last 400 years of human history basically has been liberals stabbing leftists in the back.

I needed an ELI5 really.

Liberals essentially cover for both capitalists and fascists. See, capitalist and fascist ideology are very unpopular on their own, so liberals come up with all kinds of ways to pretend that rich people owning everything is good for everyone (capitalism) or pretending that more police repression means more safety (fascism).

Liberalsm essentially acts as the pretend-friendly "facade" ideology of this unholy trio - so yes, it's simply coherent for leftists to despise liberalism.