An addition/correction to the 1500/250 FAA comment(s) above.
Until a few years ago, the minimum certificate required for the FO was "just" a commercial certificate with SIC type rating. ATP typically came with upgrade to Captain. Congress raised the minimum to ATP and 1500 hours, to "solve" a problem that had nothing to do with lack of an ATP.
The minimum flight time to get hired, however, was much higher. About 20 years ago it was possible to get a regional job with 500 hours. Not super likely, but it was happening. The reality: in the mid 80s I worked with a Navy pilot who was leaving active duty and looking for a job. He had 1800 hours - all of it turbine time (T-34C, King Air, P-3). Delta told him, "go get 200 more hours in anything. Without 2000 hours we can't touch you."
There was a comment above "how do you get to 1500 hours" when "there's only so much instructing one can do." (Paraphrasing.) In the US there is a very robust General Aviation world. There enough people learning to fly that most new 250-hour instructors can find a place to instruct and build 4-500 hours a year. Along the way they often get right seat opportunities in someone's charter twin.
"1500 hours and ATP" is new, but the reality is that with few exceptions no one was getting airline jobs with less than 1,000 hours. With 1,500 hours a competent pilot will quite likely get hired and the actual ATP training and type rating will be paid for by the hiring airline.
As hinted at above, waiting for an instructor to accumulate 1,500 hours in the European/UK market would not work. So, the entire airline, hiring, training, etc, industry has developed around the European/UK model of flying - essentially parallel paths of hobby and professional.
The USAF puts a 250-hour new pilot in training for a C-17. the Navy does the same with P-8s. So why not the same for an A320 or B737? I don't see that the European model has safety stats any better/worse than the US model. (There is a difference, the military training has a couple hundred hours of flying around in King Airs before moving into the jets...)
I can see that an MPL graduate is certainly better at reading checklists and following SOPs as that's what they did for many hours. Likewise I can see that an fATPL graduate is better at weather decisions and steep turns in a Pa-28 as that's what they did for many hours. After a couple hundred hours in the right seat their "history" has begun to average out.
As an American riding around in LoCos around Europe/UK for many years I never wondered "is there a 250-hour-wonder in the right seat? Should I be scared?" or thought "I'm crossing 'the pond' on Delta so I'm safe with a skilled crew up front."
I was in the jump seat of a C-17 leaving Baghdad at the end of my tour. Flying into Kuwait. We're cleared to land on 12R. The wind is perpendicular to us, 12-24kts. There's an Air Force Captain in the left seat (so more than four years service) and a 1st Lieutenant in the right seat actually flying (less that four years). Boom. We're suddenly left of 12L! Yes. It happened that quickly. The Lieutenant said, "What do I do?" That's not the question you want to hear. "Lower the upwind wing and use top rudder" was the answer - the same thing I did in hundreds or hours of Pa-28/C172 time...