I can provide a bit more of a qualified answer, backed up by my own observations and my research into running for office as well as my research in elections. It became an obsession after the 2020 election. With that in mind, take this with a grain of salt.

The TL;DR of the below: Democrats fall in love and Republicans Fall in Line

Broadly speaking, the people who consistently vote for Republicans and the people who vote for Democrats (i.e. their respective base) are very different. The Republican base favors control and order whereas the Democrat base favors cooperation and liberty.

Because of this, Republican politicians can get away with shit laws that are oppressive, that restrict rights, and basically allow people to be shitty to people who are different (read: white, cis, hetro people can be assholes to non-white, trans, non-hetro people). Even the most moderate Republican voter who is open-minded will still vote Republican because being "nice" is not a higher value than "order". So even though Marjorie Taylor Greene or Donald Fucking Trump are complete assholes, because they write laws that protect them, they don't have enough motivation to not vote for a Republican.

Contrast this with the Democratic base. For a moment, assume that Donald Trump, is just as disgusting, just as racist, just as misogynistic, but consistently voted progressively. You (assuming you are on the liberal side of the spectrum) would probably either not vote for him or actively vote against him. The Democrat base is also very large in that there are a lot of pockets of people who tend to vote Democrat, but only if you speak to their specific issue.

That's why Democratic candidates often have to run a very tight, very structured, and near perfect campaign whereas Republicans can literally take a candidate who's never been to a donut shop and have a gaff of him ordering a box of "whatever makes sense".

While Republicans have a consistent base that votes, Democrats do not. Democrats have to win the "middle", i.e. people who aren't politically engaged and these people often fall into three categories:

• people who are just busy either with life, family, school, etc.
• people who feel their vote doesn't matter
• people who "don't like politics" and sees "both sides as the same"

There is a common denominator of these people: they are often privileged and have never been on the sharp end of a government action and don't think it will happen to them. So campaigns have to spend time to try to convince them that it's worth voting for them.

So, to answer your question as to why Democrats can't just give zero fucks: they literally can't do that and win elections. Democrats can't keep their base if they do.

They saw the New Deal raise millions out of poverty and want to bring it back.

I'm not so sure about that