According to Osho, constant thoughts about money arise because money symbolizes power and future security; when we feel inwardly empty, we try to stuff that emptiness with money, prestige, and virtue. This thirst is endless. Only by living in the present and meditating (seeing our inner emptiness as already full) do money and power obsessions naturally fade.
We keep thinking about money because we feel empty and fear the future; being present and meditating lets that craving relax.