There is a setting for it, it's just not exposed in the GUI.

sudo pmset -a disablesleep 1

As mentioned by the other commenter, `caffeinate -s` also works, but with the caveat that a) the command has to be running and b) the laptop has to be plugged in to power. Configuring disablesleep=1 completely prevents sleep and lasts until you manually configure it back to 0.