Why does an excess contribution distribution from a roth IRA count as income since I already paid taxes on it?

Excess contributions to a ROTH are subject to a 6% penalty tax. If the excess is withdrawn before the tax filing date, then there is no penalty as long as the amount of income earned on the excess is also withdrawn. That income is taxable in year withdrawn. Distributions from a ROTH are not taxed if the distribution is from yor contributions.  Income is taxed if withdrawn within 5 years or you are under 59 1/2.