Why do we say that charges are always produced in pairs?
Charges are always produced in pairs to neutral the total charge or to maintain the total charge to be zero.
The phenomenon is due to electrostatic induction.
The word induction itself means influence without physical contact. Thus, actually electrostatic induction refers to the charging process without physical contact between the conductors.
If a charged conductor is brought near an uncharged conductor (without touching), it has been found that the later develops and equal and opposite charges on it.
Here, the green positively charged rod induces equal and opposite charges on the blue sphere. Negative charge on the near side of the rod and positive charge on the opposite side.