Question : Why am I getting taxed for my income earned abroad?

This will depend. If you are a Dutch tax resident, but you earn all your income abroad, this income may be fully tax exempted in the Netherlands. However, the main rule is that a Dutch tax resident is taxable in the Netherlands for her/his worldwide earned income, also to avoid a situation in which no tax is levied at all. The Netherlands have closed many tax treaty agreements with a lot of individual countries; such a tax treaty will appoint which country may tax which income in cross-border situations.