Why do I need to do calibration myself?

Generic profiles are usually bad. When a manufacturer creates a new model, they just take a few items from the production line and average them together:

Display panels differ quite a lot from unit to unit and change substantially as the display ages. It is also more difficult for printers, as just changing the type or weight of paper can invalidate the characterization state and make the profile inaccurate.

The best way of ensuring the profile you have is accurate is by doing the calibration yourself, or by letting an external company supply you with a profile based on your exact characterization state.