The tag says "NAPA NEW" which almost certainly means an expensive, but bottom-feeder Communist Chinese knockoff. You could haul it to the nearest NAPA and have them "test" it on their "machine". They may or may not give you another one depending on how **** they are about having a receipt.
I bet if you open it up, it's got short field coils and short windings on the armature, even though it's built with the supposedly "long" "High-torque" case. Even GM pulled those tricks on some starters.
Might be worth taking the solenoid apart to see how corroded/welded the copper disc inside is; as well as looking at the brushes and bushings.
There's lots of ways these starters can fail. Not every way is easily visible.
I've got a whole shelf-full of those old direct-drive starters. They're pointless on Chevys or Buicks, "everyone" installs a permanent-magnet, gear reduction (PMGR) starter--shorter, lighter, takes less amperage but produces more torque. The PMGR starters are mostly imported and sometimes garbage--a person has to be careful. "Good used" Genuine GM PMGR starters are wonderful when available at the Treasure Yard for pennies on the dollar.
The old direct-drives are OK for engines that don't use the Chevy/Buick nose cone, though. The Olds/Pontiac/Cadillac/Toro-Eldo/Nailhead etc. engine families were discontinued before the PMGR starters became available, so there's no OEM PMGR starters for those vehicles. There's aftermarket "mini" starters, though.