Because the underlying technology to make parts is complex in order for breadboard to work the way it does. Also because the new parts editor isn’t finished yet and thus isn’t necessarily complete. That said these tutorials may help some:
For what you appear to want to do you need to edit the breadboard svg with a program like Inkscape (rather than parts editor) so Old_Grey’s video series is probably what you want. If you like you can also post the part you want modified and one of us may be able to make the change you want easier than explaining how to do it (although if you are here for the long haul you probably want to learn to make parts).