Is it because I’m a Guest? 
Yep.
And even we Charter Members have to manually click the box every time.
There’s always the old fashioned way, which is how we did things before the software automated sigs.
You just type in your sig. The power users quickly figured out how to set a keyboard macro to do the work, while even the rest of us figured out how to cut and paste from a text file we keep on the desktop.
This is a hand-made sig.
“Bring back the old traditions, while we still remember them.” ~ Chief Joseph