How can you guys call that clean?

Look at the derailleur cage and pulleys. Take a look at the rollers on the chain. The rollers are silver/gray. That's a layer of dirt/crud covering them. On a new chain? I'll bet that the new chain was not cleaned before installation, and it's rapidly picking up dirt from the derailleur pulleys and off the road. This much crud can lead to poor shift performance. What do you think the top pulley looks like? If full of crud, may be it's isn't rolling, and sliding as it was designed to.

If the cog tooth was damaged/bent before the cassette was initially installed, it could have been returned as defective. A little time cleaning the existing parts, and inspecting both the parts being reused and the new parts would have detected a defective cassette cog. After cleaning the existing parts, and inspecting the new parts to be installed, then installing the new parts, time should have been taken to adjust the rear derailleur and the shifter for proper functioning before the bike was put back into service.

Most mechanical problems on bikes are caused by the rider/owner, and are the result of inadequate mechanical ability, neglect or abuse. They are not caused by parts that suddenly go bad after installation, although parts do wear out and break even on the best maintained bikes.