Because the useful volume of a laptop is somewhat limited and in the cabin you can ensure placement against the skin of the aircraft. In the baggage hold it will have been scanned better and by a process that is less susceptible to compromise by a single person, it will have been placed in a somewhat random position inside another container with a fragmentation blanket around it, etc. Yes, you can certainly bring down an aircraft by putting a bomb in luggage, but a lot of effort has been put in place to prevent this exact thing.
In some airports, your technology will even be removed from your luggage by a helpful and diligent TSA or airline employee, and they will take it back to their own home for a very thorough functional check that may take years to complete.
CNN, Chicago Tribune, and ABC News ran a few stories about this "extra-special luggage screening" around 3-5 years ago, which I won't link because of autoplaying videos and/or slideshows.
In most modern western airports checked bags are scanned using a CT scanner rather than a 3 image x-ray that you see used for carry-on bags. This gives a much better image with more spatial clues regarding the contents and makes the resulting image much easier for automated detection assistance. Checked bags are also often passed by canine units for explosive detection.
I've read that too. The scanners for luggage are much better than those for the security lines. They might even use machine learning to detect things. I don't remember the source. sorry