Why are US airlines so bad?

In general, US carriers seem much more cheap and stingy than their European and Asian counterparts with little difference in fares. First of all, complentary food is nonexistent in coach in the US (not even on 6 hour cross country flights) while it's commonplace in other countries (example: China). Second, US carriers' planes are always in terrible shape. The plastic panels are yellowed, and badly scratched and scuffed up, seats are heavily worn and battered, and all the furnishings very dated and look like they havn't been updated in 20 years. International cabins are OK but flying domestic on, say, United, is like flying a budget carrier overseas but without the lower fare.

The US air carrier are the safest in THE WORLD. IF I remember right, there were ZERO deaths last year in the US by commercial airliners.

US airlines are private businesses trying to earn a profit. Most of the carriers you refer to are supported by government funds.
The condition of planes is somewhat related to their age and usage about the cosmetics.
What is included in the fare varies by ticket class and airline.

From Beijing (PEK) to Shenzhen (SZX) Thursday, 08 March 2018 - Thursday, 15 March 2018 roundtrip on China Southern Airlines as China's largest including tax shows US$660 as Economy fare. It includes 20kg checked bag and 5kg carry on.
1,937 km= 1,204 miles
That is close to NY to Miami in distance, on American Airlines, US largest on the same schedule is $258 round trip plus $25 baggage fee for a checked bag and can add a $10 meal for $293 Round Trip

If you can show where it is less than half price buying it locally in China with cny, you do that.
Otherwise, I do not see where the fares are lower in China.
I believe them to be higher in Europe than the USA, unless ultra-economy RyanAir stripped of services.

I do believe air travel is lower priced in the USA, but also know about government support of airlines in other countries.

Show an example and I will investigate it.

Customer service on US airlines is bad because US customers do not want to pay for it. They might complain but they will always pick the cheapest fare.

I don't know what airlines you fly. My last two flight had an usual smell (probably the same plane = departure & return). It wasn't a bad smell… It was a good smell… Just an usual smell.
- the flight attendant called it a "new plane smell".

Some seats/airlines do show some wear and tear but most don't.
The most run down plane I've ever been on was a China Southern flight domestic within China. SCARY!

The US does have older planes but they are slowly phasing them out and replacing them with new ones. American, Delta and United have added new planes such as the Boeing 737 or Airbus 321 on domestic routes and Boeing 777-300 on international routes. It really depends on what route you are flying.

The replacement may not be as noticeable or as fast as you would like partly because the airlines are opening up new routes and the fact that these airlines have a large fleet to replace. Also US airlines tend to let their aircraft fly longer than some European and Asian airlines. Know that some of the really nice airlines may be receiving some type of government subsidy where the government is backing them up by giving them cash. Therefore they have money to buy new planes and give the nice service they do.