I'm no aerospace doctor or nothin, but I'd think the pilot would be expected to stay on a very strict schedule. If they arrived at the intended airport minutes ahead of schedule they might have trouble landing when another scheduled flight is trying to land or have to awkwardly circle the runway until there is clearance.

I'm sure the airlines want those throttles set to the most fuel efficient setting that gets the plane there safely and on time. Fuel is expensive.

As the other response noted, arriving early can lead to headaches, but for other reasons - for example, slots at the gates are timed, so arriving before the previous plane departs won't work well. Of course this doesn't always apply, sometimes the gate is just sitting empty already.

Also, airlines have padded their schedules for years to give themselves some leeway in case of unexpected delays in taking off, landing, minor issues with the plane, etc. I assume this is a relatively small amount, like 5 or 10 minutes, but I really don't know. But it does mean the flight will show up early frequently when things are normal.