You need two things to be a good pilot: experience, and natural ability (well three things, training). The 1500 hour rule reduces the talent pool that even start flight training, since talented kids have lots of options, and staring down the challenge of finding 1500 hours makes a career in finance or coding look pretty good by comparison.
That means the airlines have a smaller talent pool to hire from, even though the pool is more experienced. And airlines have the choice: either shut down routes and sell airplanes because they can’t find good pilots, or let the bottom of the barrel 1500-hour kids in the cockpit. So what happens? If you can get 1500 hours you’re guaranteed a job at some airline even if you’re terrible.