Why the internal resistance of a voltmeter is very high ?

A volt meter measures the potential difference between two points. If its internal resistance was low, a lot of current would flow and this would reduce the potential difference between the points and immediately giveyou the wronganswer.

If the internal resistance is high, and it isusuallyvery high indeed, then very littleor no current will flow through the meter. That enables it to give a more accurateanswer.

Ideally, the internal resistance should beinfinityhigh, so that no current what so ever can flow through it. In cheap voltmeters, it is harder to make the internal resistanceinfinityhigh and so with very high voltages, a larger amount of current than normal might flow and the voltage reported is then wrong.