It's a throwback to the industrial revolution when factories employed very uneducated peasants. They didn't know multi-syllable words like protrusion, receptacle, etc, so simple words they would understand were used. The male part goes into the female part.
Actually, this is where we get our swear words from. Penis, vagina, intercourse... all multi-syllable words used by the educated. Cock, wise and beautiful woman, fuck, all single syllable words used in every day life by the peasants, and given a negative connotation by the educated so they could distance themselves from the poor.
It's also the same reason why
this tool used on a lathe, is called a
dog.