I agree with the sentiment but it's more like:
Dems: There's no way we'll get single payer universal healthcare passed because other humans exist and they have the right to vote, but we can do the ACA and try again in a few years
Leftists: Stop fucking compromising with facists you capitalist piece of shit. Just make them do what we want. I'm never voting for a Democrat in my entire life ever again
You have to be a reliable constituent and help them get elected. Young people, progressives, leftists, whatever aren't reliable enough and don't vote in numbers significant enough to shift the Overton window significantly in a short time frame. Since they aren't reliable voters the candidate has to go farther to the right to get the votes they need to get elected.
Slow progress is the best you can hope for when you're hoping for extreme change to a system, and you can't stop because if you do a single Republican win can undo years of progress (e.g. Trump with the supreme court nominee and Roe).
Another issue is progressives are really spread thin on issues from civil rights, to economics, to foreign relations, to gun rights, you get the picture. This big tent brings in a lot of people, but some people get pushed out when you bring them in. For example many Latino voters are Catholic and are anti-LGBTQ and anti abortion. They have to decide if they care more about the inclusion the Democratic party brings, or the abortion restrictions the Republicans bring.
All of that to say progressives a often single issue abstainers; they will abstain from voting over a single trigger issue. The more issues a candidate has to support the less likely it is the candidate will support that voters trigger issue to the degree the voter wants. Now that voter is staying home and not voting, which leads to the point I made earlier.
Also we have made a lot of progress. Gay marriage is legal now, people are much more aware of racism and other discrimination and we're taking active steps to combat it, the ACA has helped with medical coverage a ton, the general population are aware Palestinians exist and Israel isn't the good guy, we've installed an amazing amount of green electricity power plants, bike and public transportation are things people want now, and many other incremental improvements.
Things aren't perfect, and some things are still fucked, but we're getting better. We can't stop because it isn't going exactly the way we want it to.