I am an Oregon resident at college in California, I earned my income in California in 2018. Why do I have to pay Oregon taxes?

[DDollar]

Because, as you said in your question, you are an Oregon resident. As an Oregon resident, all of your income - regardless of what state it is earned in, is taxable by Oregon.  Since you will also be paying California tax, Oregon will give you a credit for tax paid to California.