A couple of other factors to consider along with the above points.
1. No cadet programmes in USA. By and large there are 2 ways of getting a job with a major or national airline- a) civilian experience through GA and thence commuter airlines, b) military. The civilian pilot will typically start as a flight instructor or rhs in a small twin, the military pilot will have far less hours but will probably have time in a large transport or fast jet. Either way it is extremely rare to even be considered by an airline in the US (market forces permitting as well) with any less than about 2500 hrs. This is sometimes for a position as FE or SO.
2. Very large GA industry in USA. Training is cheaper, and therefore attracts more students. More students, more instructors. Corporate flying, small charter companies, cropdusting, police work, sightseeing etc. GA is HUGE in US compared to Europe, and therefore when large airlines are out selecting new hires, they have a larger pool of experienced pilots to choose from thus negating the need to hire 200-hr cadets.
3. Low popution density. Compared to europe and even more so asia the population density dictates that there are fewer large jets per capita in US than in europe. There are far greater RJ's, turboprops, piston twins and so forth. You are probably used to seeing an MD80 or B737 as an "entry level" type of aircraft to get a job on. Due to the above factors crews on those types in the USA have typically flown a C172/Navajo/Caravan/B1900/ Metro/Dash 8/ATR/RJ etc beforehand.
4. Defined career path. Most likely a function of the numerous aviation cycles a person has been through. A 55 year old MD-80 F/O at one airline may have previously spent 25 yrs as Captain on a B737 only to see his airline fold or be bought out by the new one, thus forcing him to the bottom of the new companie's seniority list or out on the street to look for work as a "new hire" once again. Only the very lucky few pick the "right" company and are able to enjoy consistent career progression until retirement.
Hope that helps.