Because they were modeled after interrupts, and can't be changed for historical reasons. (E.g. any code which relies on the atomicity of a signal handler with r

Because they were modeled after interrupts, and can't be changed for historical reasons. (E.g. any code which relies on the atomicity of a signal handler with respect to its context thread would break.) Signal handlers were always meant to be "top-half" interrupt handlers -- set a flag or put something in a queue for the main thread to process. signalfd(2) formalized this usage.