That is your opinion, and based on your experience in your cohort, and additionally does not really address what "more left" actually means. There are many different types of people that vote Democrat at many different stages of life, in many different industries and in different states/areas where certain things have more support than others. And that's before we even start breaking things out to being "left" socially or fiscally.
I don't think the answer is "quite simple" at all, at least not in the way you explained it.
Couldn't it also just be that there's a lot to keep track of for our current government (of laws and making/updating/enforcing them)? As society grows, you inevitably will have issues that fall through the cracks if administration doesn't also grow accordingly. Maybe they haven't gotten to updating this $2,000 thing because they're focused on other more important things? Hell, I doubt there's a single politician, that knows at the top of their head, all the laws and policies we as a government have in effect, along with all their nuances...
And then the other problem is that the majority has to come to agreement for things to actually change. This is often a slow process. Sometimes you have politicians trying to change stuff like this, but they alone don't quite have enough power, and it takes time for them to convince others that it needs addressing, let alone the proper way to address it.
I notice we have not had Bernie Sanders on the Presidential ticket, despite two runs; and there is surely some reason for that.
Possibly the voters have identified that those candidates will not gain sufficient independent or Republican support to actual win in general elections.
In any case, it seems likely to be a lot more nuanced than you're suggesting.
Assuming that the primaries represent who will actually win a presidential election is as much of a mistake as assuming that polls are accurate when it comes to election day.
On the other hand, the far right thinks anyone left of Trump is a socialist and will vote Republican just to try to keep away socialism.
The democrats in office simply do not represent their constituents. The democrat-voting population is farther left than the democrat politicians. However, outside of the more radical democrats like Sanders and AOC, most democrats have been bought by the wealthy just like the republicans.