Why is it that so many things can be taken on an airplane but can't be checked in?

An aviation major to answer this question. The reason why some things can be taken on the plane but not checked in is because the things you take on the plane are managed by someone, while the things you check in and put in the luggage are not managed by anyone.

Shown below is a diagram of the internal structure of an airplane's baggage compartment, and we can see that the cargo is stacked right in the cargo hold, very densely. And there is no one to look after this cargo during the flight, so in the event of a spark or something like that, it's very likely that a large piece of cargo will ignite.

While these cargo bays are usually equipped with automatic fire suppression systems, fires can still cause problems such as cargo loss and airplane delays, bringing huge losses to airlines.

So things like batteries can't be put in the cargo hold, but have to follow the people on board the plane, is because batteries can easily short-circuit and cause spontaneous combustion, and in some cases may also be due to the plane's bumps lead to battery damage, causing combustion or explosion. For example, the previous Samsung NOTE7 because of frequent spontaneous combustion accidents are prohibited to carry on the plane, this is the main reason.

And in fact, although the spark is terrible, but when the spark just up is not serious, as long as the timely detection, targeted to extinguish the source of the fire, will not cause greater losses.

So it's relatively safe to say that battery-type items are a lot safer if they're brought into the cabin of an airplane.

On the one hand, it is because many people will put the battery items in proper storage, such as in a softer place, rather than in the cargo hold as up and down bumps, collision; on the other hand, once the battery has any abnormalities, people will be the first time through the odor found, and can take measures to extinguish the fire, to protect the safety of the aircraft.

For example, the picture below shows a sudden battery fire during an airplane flight, which was immediately detected and extinguished, while if the fire had occurred in the cargo hold, the best case scenario would have been for the fire suppression system to extinguish the fire after drenching all of the cargo, or for the fire to be out of control, resulting in a catastrophic accident.

That's why items with batteries aren't allowed to be checked in, but can be carried on airplanes - it's all about safety.