Question

why is voltmeter always connected in parallel and ammeter in series, why not vise versa????

Answer 1

When instruments such as voltmeter and ammeter are connected to a circuit, we want the desired measurement readings as output AND least possible interference in the circuit due to the instruments (due to the resistance of the instruments to be exact) .
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In case of voltmeter:

We want the measure of potential difference between two points in a circuit as output (Voltmeter reading).
So it is connected in parallel to the two points (which will still be same potential difference).
Plus we want the circuit to behave normally. So the voltmeters generally made with very high resistance so that current doesn't pass through the parallel connection and goes through the actual circuit (path of least resistance).
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Similarly,
In case of ammeter:

We want measure of Current passing through the circuit as output (Ammeter reading).
So it is connected in series within the circuit of which, current is to be measured.
To avoid effect of instrument resistance on the current flow, Ammeters are generally made with low resistance.

Answer 2

Ammeter:-

An ammeter is just a converted Galvanometer and measures the current flowing through the circuit by showing proportional deflection. An ammeter is formed by connecting a Galvanometer with a low resistance in parallel.

$\bigstar$So the Ammeter is connected in series with the circuit elements and measures the current flow in a circuit.

Voltmeter:-

A Voltmeter is also a converted Galvanometer which measures the amount of Voltage (or) in other words the potential difference accross two points in a circuit. A voltmeter is formed by connecting a Galvanometer in series with a high resistance.

$\bigstar$So the Voltmeter is attached in parallel in a circuit and measures the voltage in a circuit.

And if this is not maintained then there is a high chance for the ammeter to get damaged due to low resistance.