Are there any numbers on that? Sure, there are lots of people who want to be police officers, but that's not the same as being qualified. As far as I can find, unemployment rates among officers are extremely low, there are many unfilled positions, and officers who moved have little difficulty finding new jobs. That all usually indicates a tight labor market.
It's possible police academy is unnecessarily hard or something, but I suppose you could say that in any field (perhaps Google's hiring standards are too high, or med school is too hard).
Entry level police positions are typically very competitive. In some jurisdictions (in my area in NJ specifically), you simply cannot get hired without having connections to existing or retired officers. Getting fired is virtually unheard of. Local Colleges are filled with criminal justice majors will who never make it to an academy. The benefits, pay, power, and perks make it the ultimate job, if you can get it.
It's possible police academy is unnecessarily hard or something, but I suppose you could say that in any field (perhaps Google's hiring standards are too high, or med school is too hard).