In part, because the political “dialogue” in this country isn't a dialogue, it's a propaganda battle where on every issue it is a battle to control the framing, largely with the aim of mobilizing each sides base while alienating the other, not coming to some kind of common consensus.

In part, because the left viewpoint on the economics is not the dominant view of either major party; the (still, arguably barely) dominant faction of the Democratic Party is economically center-right.

> But for some reason people on the left seem to act like this contention doesn’t exist. They just talk about how terrible the border and ICE is; both of which have been awful for many years

Even given the preceding, that's not really true: the left (and Democrats more broadly) do offer much more specific policy criticisms than that, and those didn't all originate under this President. (Though under the immediate preceding one, those positions were often shared with the President though not the Congressional majority, see DACA/DREAM Act.) Obviously, they've become more intense and higher priority with a hostile administration engaging in policy more hostile to their desires.

> It’s nice that people care all of a sudden

People on the left don't care “all of a sudden”, though obviously the policy context has changed all of a sudden which has shifted the tone and focus of criticisms.

> but name-calling doesn’t address these rational claims of the opposition.

Electoral politics largely isn't about rational debate, and trying to make it so is often counterproductive, even if you are winning with the people that are listening for rational debate.

> It sounds like if we can just actually take the economics seriously for once then it would eliminate any decent ground to defend the immigration enforcements.

The left isn't against immigration enforcement so much as it is against the particular policies being enforced and the methods of enforcement.

But “decent ground” isn't, even if it should be, often what wins policy debates.