Question

Why is ammeter always connected in series, not in parallel?

Answer 1

Ammeters are tools used to measure the current in a circuit. The ammeter is connected in series with the circuit so that the current to be measured flows directly through the ammeter. The circuit must be broken to correctly insert an ammeter.
Because Voltage is the difference in potential between 2 points.

So if you connected a voltmeter in series all you are doing is measuring the difference in potential across a strand of wire which will be zero (or very near). To measure voltage you connect the voltmeter in parallel so that you measure the voltage from the - to the + or the voltage drop across a load.