There are many free ways you could have prepped for the tests. There are many free tests available on websites, as well as resources from places like Khan Academy. Your local library would have al the necessary books for prep as well.
3.8 GPAs are not created equal and the test scores are one facet, a major one, that helps determine your ranking so to speak when comparing with others. At a school like UCI with over 100K applicants, over 70% of the applicants will be rejected and I’m sure a good number of them are also perfectly ready for UCI also.