There are two reasons for airlines to choose pilots with thousands of hours.
First, there is a fairly recent regulation that all airline pilots (with heavier jet airplanes) have a license called an Airline Transport Pilot certificate. It requires at least 1500 hours of flight time in most cases. So that’s the minimum number of hours in the United States. Europe and other countries do not have this requirement.
Second, hours are a rough approximation of experience. Some hours are better than others in the airlines view so they’ll choose somebody who has flown a lot of hours as a regional jet pilot over say a pilot who has a lot of hours crop dusting.
In the old days, the airlines thought a lot of hours made a better pilot and they would give those pilots who racked up a lot of hours, preference in hiring. For a long time, they also favored military experience over civilian.
Things have changed a lot in the more recent past. The airlines have finally discovered that the relationship between hours flown and ability are not correlated very strongly. So they now set a fairly low bar for a pilots number of hours and instead look at other more important dimensions.
Once a pilot meets the minimum qualifications of hours, licenses and education, they are then screened for intelligence, trainability, compliance, judgement and mental stability. A few years ago I heard that they interviewed about 30% of the applicants and chose about 1 in 10 of those so the chances of getting hired were pretty low.
I fully support the newer methods of selecting pilots. I’ve noticed that the new copilots are sharp and interested and driven and very, very good at what they do. It’s really been a big improvement in my observation.