Not everything is included in the plan. Computer coverage often extends to connected peripherals in addition to the mouse and keyboard. Service levels can also vary. As a bonus, some services even offer replacement equipment if it needs to repaired for the same problem multiple times. Extended service plans are not cheap, though.
Manufacturers typically charge considerably more than third party providers.
Service plan pricing is often highly negotiable, however. If you want an extended service plan for all your equipment, which can cost several hundred dollars, it's worth shopping around. Equipment repair services and other stores are alternate sources you can turn to for these plans. Nor are you required to buy one at the time of purchase. In most cases, you simply need to sign up for one 30 days before the original manufacturer's warranty expires.