Because that isn’t how parts are created and because parts editor is not finished (and is thus limited.) If you really want to make parts (which is quite difficult) these two tutorials apply to current Fritzing versions and we are happy to provide assistance if you have problems. For perf board you are looking for top view parts (which are modified for use on perf board.) A google search of the form “fritzing part part number top view” (where part number is the name of the part you want.) will find anything available. Here are the pointers to the tutorials:
I lately learned there aren’t links to the videos in Old_Grey’s tutorial so you need to do a google search for the title and then they come up on YouTube.
Part-creation-howto-part-1-breadboard-and-pcb .
Peter