Why are certain types of door locks called 'deadbolts' ?
A deadbolt is a type of locking mechanism that is installed on doors to provide an additional layer of security. Unlike regular door locks, which typically use a spring-loaded latch bolt that can be easily manipulated, deadbolts use a solid metal bar that extends into the door frame, making it more difficult for intruders to force open the door.

The term deadbolt is believed to come from the fact that the locking mechanism is dead when it is not engaged, meaning that it is not spring-loaded like a regular latch bolt. Instead, the deadbolt is locked and unlocked manually using a key or thumb turn, which moves the solid metal bar in and out of the door frame. Because of this solid bar, deadbolts are generally considered more secure than regular locks and are often used as a secondary locking mechanism in addition to a standard lock.
Do deadbolts have any negatives? Why don't all doors have deadbolts, if they are this much safer?
While deadbolts provide additional security compared to regular locks, they do have some potential downsides. One potential drawback of deadbolts is that they can be more difficult and time-consuming to operate, especially in emergency situations where quick exit is necessary. Additionally, deadbolts may require professional installation or modification to the door, which can be an added expense.

Not all doors have deadbolts for a variety of reasons, including cost, ease of installation, and personal preference. Some homeowners may feel that regular locks provide sufficient security, while others may prioritize convenience over added security. Ultimately, the decision to install a deadbolt or not depends on individual needs and circumstances.