Do we have a good idea as to why this has flattened out?

Yes, we have a good idea as to why. There are a few key factors involved starting with a shift away from full time employment and towards part time and gig work. This means that the number of people working full time jobs with the protections they provide has not gone up even though the population has gone up over that time. These workers are more precarious, experiencing less stable hours, unpredictable income, and easier firing processes. This makes people feel less secure and accept worse conditions. People with precarious employment may be less likely to report wage theft for example, which to be clear is more than all other types of theft combined.

Maybe the US slowed its population growth?

I'm no economist but I imagine it has to do with the wealth hoarding that our elite is engaging in.

I believe by most metrics the ultra-rich are holding onto very little cash, most of it is invested (which means some company is spending it to hopefully make more). For example, most of Musk's fortune is in Tesla and SpaceX, which are employing people and selling things.
I don't think shortage of cash for businesses (or even consumers really) explains the stalled labor market. Uncertainty about terriffs and fed rates are probably more indicative (and I think it's hard to argue that those are directly caused by wealth inequality).

It is also going to drop soon. When immigrants, illegal or not, are forcibly removed, you don't only remove the low-level jobs required to keep e.g. farms/factories sustainable (general US populace is unlikely to pick up this kind of low-education/low-wage jobs), which forces these companies to close. On the other hand, fewer people in town also means lower spending power, which means that fewer (e.g. food) businesses are sustainable, and this again leads to the closure of companies. The increase number of companies closing will also mean greater unemployment.

Just going by this graph and headlines over the last half-decade, probably still COVID hangover. You can see it did something similar in the early 90's and early 2000's.

At some point going forwards, blowing up all the US's trading relationships is going to have an effect, as well. And maybe the AI bubble will burst.

Economy is fucked.

Aside from the wealth concentrating in the hands of a few, there's also noting that it's "working full time". More people are stuck into part time or multiple part time jobs to make ends meet.

Unemployment in the USA has only gone up a small amount:

But full time employment has fallen while part time has risen:

It's also important to remember U-3 is a rate, not a count. Labor force participation is closer to a count, but has to use calculated values over longer periods, making it more of a rough estimate.
- Also that U-3 is a very specific metric that is notorious for undercounting.

We are about to see unemployment numbers like we have never seen. In my opinion of course.

gig economy

I mean I have not worked since 2024 and my state won't allow me to inform them im still unemployed and looking for work.

"Won't allow" you?
- yeah there is a system in place to report it even when you are not getting benefits but at one year it won't let you anymore. thats as long as they are willing to keep track. even though they have an automated system to do it where I put in the work to register my status.

jobs pay a lot less now, BC of inflation, so there is more money for employment. also there's a lot more people do that makes job numbers go up

Aging population.

March 2023 was when GPT 4 was released. Maybe it’s coincidental, but I think the AI hype has some factor in it. Companies got an excuse (a bad one) to lay off their staff.

There was a recent article somewhere here on Lemmy finding that this trend was pretty uniform across sectors with different exposure to AI, so maybe it's not that.

If you look pre-Covid, you'll see that the trend line goes back to 2020. Various reasons for the flatter line:

Retirement. A lot of boomers are hitting retirement age and there aren't enough zoomers to fill the void.
Interest rates. Inflationary policies due to Covid have made money cost more. This has lead to previous economic growth strategies no longer becoming tenable.
Tech collapse. The tech sector in the USA has hit the right side of the S-curve for a lot of different technologies. This has limited growth of one of the major engines of the American economy.
Remote work. Better remote work tools have allowed people to work wherever. Along with this, companies can hire people from a larger geographic area to perform tasks. A lot of hiring is happening in lower cost of living areas, including outside the USA.
Gig economy. More people are relying on the gig economy to make ends meet. These are not full time roles, so they aren't counted here.

What do you mean. It's a graph of working people. If people there aren't more people working, it's either that people are missing, which is unlikely with a country that big (and so far open to immigration) , or the economy can't pay them.

As you can see by the struggles after 2008 and during covid.

It's the economy.

Its sorta funny because just before covid we were due for a correction but it sorta short circuited the whole thing and this kinda shows how its limped along since.

I don't believe in capitalism, so I'm against charts.