Because doing that requires making an "extension cord" with two male ends on it, a thing that is a dangerous risk to life by it's mere existence, DON'T DO IT.
There's a reason you can't buy one like that. That reason is that sooner or later somebody is going to pick up one end, not realizing that the other is plugged in, and somebody will get electrocuted.
You want to supply power to your rig via a 15 amp circuit, buy a proper 15 amp to 30 or 50 amp adapter and plug in that way. The lives of both you and your family and all your friends are in your hands.