Quantity Theory of Money Explained

The Quantity Theory of Money is valid in the long-run; however, more relevant in determining where we are in a cumulative process of price movements is the QTM in conjunction with the Wicksellian Natural rate of Interest.
Yet my biggest question is, why do we have an exogenously determined money supply?
Academic studies that support the Quantity Theory of Money:
https://www.researchgate.net/publication/4886300_The_Quantity_Theory_of_Money_Evidence_from_the_United_States

https://core.ac.uk/download/pdf/236413141.pdf
https://link.springer.com/chapter/10.1057/9780230280854_35
Fed Data on the Money Supply:
https://fred.stlouisfed.org/series/M1SL

Fed Data on the Velocity of Money:
https://fred.stlouisfed.org/series/M2V

Fed Data inflation:
https://fred.stlouisfed.org/series/PCE

Knut Wicksell and the Natural Rate of Interest:
https://political-economy.com/natural-rate-of-interest-wicksell-price-stability/

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