The keypad on your microwave breaks so you can either pay a ridiculously inflated price for a replacement part thanks to a proprietary design, or just buy a new microwave even though the other parts of your old one probably work fine. Oddly though your $5 TV remote never seems to have that problem in spite of frequent use and extremely low manufacturing costs. Some things really are just designed to fail and it's not always because making something durable would cost more.
I don't have a good answer for why laptop manufacturers don't use a modular construction, As it would indeed make upgrading easier. That said, you can upgrade a good many parts, such as the memory, the processor, and the harddrive, greatly extending the useful life.
As to why the remote lasts forever and the microwave buttons don't, it's a simple factor of the remote buttons being made with a different switch technology due to the different expected conditions. In a kitchen, everything has to be sealed, so they use a 'bubble switch', a metalized piece of plastic which has a tendency to break with repeated use. Compare that to the remote, which has a traditional momentary push button in it. It's not intentional, but it is a consequence.