I'm not sure about the present justification, but VAT is an alternative to sales tax, which has existed for a very long time, while Wikipedia says income tax as

I'm not sure about the present justification, but VAT is an alternative to sales tax, which has existed for a very long time, while Wikipedia says income tax as we know it started in 1798, so there may be an aspect of historical legacy. Although both VAT and income tax require states with extensive administrative capacity so I guess if there was political will to do income tax only it could be done. Maybe it is the politics of progressive vs regressive taxation? Or just that doing income tax only would mean very high income tax and high tax evasion?