Because capitalism is a system in which the rich get richer. This preferential accumulation of wealth where wealth already is is an inherent feature of capitalism.
The same kind of phenomenon can be seen in the population of cities for example: the more people there are in a city, the more other people are likely to move there.
If you want to know more about this, see, e.g. Picketty (https://en.wikipedia.org/wiki/Capital_in_the_Twenty-First_Ce...)
Also, profound inequality is bad for growth (The IMF says so, https://www.imf.org/en/Publications/Staff-Discussion-Notes/I...): "when the rich get richer, benefits do not trickle down.".
So given that there is a runaway feedback loop that make the rich get richer faster than the poor get richer, and given that profound inequality is detrimental to the economy at large, it logically follows that there must exist some scheme to redistribute a part of the wealth of the extra-rich to the rest of society.
Taxation is (or should be) such a shame.
Note that there is no moral argument here. No finger pointing, no entiltement. Just a systemic view of things: it is detrimental to all to let a runaway feedback loop increase inequalities.