Why require a minimum of 1500 hours to become an airline pilot?
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.