Why is the voltmeter connected in parallel but the ammeter connected in series?

The Voltmeter is used to measure the electrical potential difference between two points...

so it's no use to connect it in Series , because the electrical potential difference between two points at the same line is zero ==> the readings will always be zero.

The Ammeter is used to measure the electric current in a circuit...

so it's no use to connect it in Parallel , because you want to know the current flowing in this wire.

note

the internal Resistance of the Ammeter is very little

and very high for the Voltmeter --> so they will not affect the circuit.