That, but also:
- Low quality parts
- Bad build quality
- Incompatible & proprietary motherboards, coolers and power supplies
- Bizarre part choices, like severely underpowered cooling and power delivery or using just 1 stick of RAM when 2 sticks would be significantly faster with about the same amount of money
- Windows installation filled with unwanted software and/or outright malware, like McAfee, which can have a double-digit % effect on game performance.
- The purchase process can involve dark patterns, like cleverly hidden support contracts with a monthly fee
The PC hardware channel Gamers Nexus has been reviewing prebuilt systems for past few months [1]. None of the reviewed computers have been great, and the ones from big manufacturers (namely Dell and Alienware) have been the worst of the bunch.
[1] https://www.youtube.com/playlist?list=PLsuVSmND84QuM2HKzG7ip...