The U.S. economy is geographically spread out and
enormous. However, due to various historic reasons, the population is mostly focused at the coasts, and for other historic reasons, the north-west and north-eastern coastal areas are the wealthiest and densest. More people and more money means more demand and higher prices.
In the rest of the country, which is fairly sparse, you can find high standards of living, minus some of the extras you'll find in the rich, dense, coastal areas (extras usually in the form of various culture and activities). However, due to the lack of local demand, the prices are lower.
This creates hugely variable economic conditions on the order of the differences between London/Paris and Budapest.
Housing, like anywhere, dominates living expenses after taxes $30k/yr income looks more like $22k/yr. There are single bedroom apartments, not fantastic, but pretty nice, around where I live that are $2k+/mo. Now you have no money for food, transport, clothing, hobbies, etc. You can find cheaper places for around $1.5k+/mo (there are no housing options here under $1200) but that still leaves you pretty thin with $4k left to spread across the entire year for other necessities.
On the other hand, in Omaha, Nebraska, I found some comparable apartments at $500/mo. Which gives you a little more than $1k/mo for the rest of the year. You'll probably blow through half of that on food and half of that on car expenses (cheap housing in the U.S. implies almost no public transport options) and now you still have no money for savings or hobbies.
The median income in the U.S. is ~$54k/year (about the same as Europe but a hair higher). But that also means children, a couple income earners and so on.