Why require a minimum of 1500 hours to become an airline pilot? In the United States, under parts 61 standards for pilot certifications, for a person to apply for an airline transport pilot certificate one of the aeronautical experience minimums is a minimum of 1500 hours total flight time. Most ICAO compliant countries require similar numbers.  My first question on this would be why so many? It’s true that you want airline pilots to be experienced and no doubt I would not expect an airline to make someone a captain of a large, turbine airplane without a minimum of say 2000 hours with a lot of that being hard instrument and weather time but why require a first officer to need a minimum of 1500 hours for an ATP?
I’m aware prior to 2014 that the FAA allowed part 121 first officers to fly with only a commercial pilot’s license for the right seat and the 1500 hour rule only came about after the Colgan air crash in 2009.  People argue that this is what is resulted in only one commercial airline fatality over the past 10 years.  Still I wonder why an arbitrary number 1500 was chosen for an airline transport pilot certificate. Now I am also aware that this is not always the case. For instance graduates of approved part 141 schools can get a ATP with only 1200 hours TT with a aviation related associates degree, 1000 hours TT with an aviation related bachelor’s degree and military pilots with 750 hours TT can apply for an ATP directly.  My only thoughts would be, if part 61 training is so terrible that it requires a person to put in 1500 hours in a Cessna prior to applying for an ATP, what makes an aviation bachelors program superior or military flight training as compared to it, and why not simply train pilots under parts 61 to the same standards as the military does?
EDIT
Some other thoughts on this topic:  Well the military does allow people to operate large turbine powered aircraft with FAR less that 1500 TT, they’re in a position where they simply have no other alternative.  You can’t give a fresh 2nd Lieutenant 1500 in T-6 or T-38 prior to dispatching him/her to an F-16 squadron somewhere as its simply not affordable with trainer aircraft costing several thousand an hour to operate and training budgets already tight.  So they’re sent off to a active unit with 200 to 300 hours total time.  The downside is that, even in peacetime, military aviation units suffer horrendously hi accident rates as opposed to their Part 121 counterparts.  As I understand it, the USAF alone had 39 class 1 mishaps in 2018.  It’s an unfortunate fact that, as much as primary flight training could teach you, it’s only a license to practice your craft; flying is an artform and takes a lifetime to master and there are a lot of dangers out there.  That being said, mass hours alone are really not good experience.  It requires APPLICABLE experience to matter.  An aviator train did a small part 61 flight school in central Iowa and logs 1500 hours flying Cessnas locally as a CFI is nowhere near ready for the rigors and stress of an airline pilot, but a pilot with 500-600 hours TT with complex, instrument, and multi-engine turbine time in high density airspace around major cities is far more likely to success here.
It bothers me that these kinds of regulations are written this way, without any thought to the practicality of it. It’s also strange that way, since, most of the FARs are very practical and based on well researched information as well as accidents in the past.  It also bothers me that it causes new pilots to bear an extreme burden to build those kinds of flight hours for a career, which may or may not be beneficial to them. I’m just curious what the FAA’s thought process was in recommending the scheme
The 1500 hours is requirement for ATPL (airline transport pilot license), which is needed to command a multi-member crew. But in most of the world the CPL (commercial pilot license) with a 250 hour requirement is sufficient to fly as a co-pilot (F/O), and that used to be the case in the USA as well. And those hours include any spent getting the private license and instrument, multi-engine and type ratings.
My understanding of why the USA started requiring ATPL for F/Os as well is that it was only indirectly related to safety and had most to do with job market. At the time it was introduced the large supply of aspiring pilots led to very low wages in the regional airlines, and with that tendency to also overload the pilots and save money across the board, ultimately affecting safety. Raising the requirement aimed to limit this by making pilots more scarce.