if the reason isn't systemic racism, than what is it?
Blacks are much poorer than whites, are less likely to have stable families, are less likely to have high-quality educations, etc. This is caused mostly by past slavery, long-time government oppression, and past racism. A freed slave had no property or education, so they couldn't afford to educate their children or give them much property. Furthermore, they were often basically forced into a pseudo-slavery system by the government. Then due to poor education and racist laws, the next several generations found it very difficult to accumulate much wealth and very easy to have any wealth they did accumulate taken from them. Government, courts, and society were constantly pushing them back in an ever-deeper hole. Even a couple of generations ago, after most of the racism written into law was gone, societal racism was a drag on these people.
However,
now the direct racism is mostly gone. There are still a small number of racist people in powerful positions, but IMO it's a minor factor. If you reformed every single one of these people, I don't think you'd see much difference in society. The problem is that many black families were pushed into a very deep hole, and they just haven't had time to dig themselves out yet, even though society is no longer still dragging them back into the hole very much. Generations of oppression isn't going to be undone immediately, even when the oppression stops.
The justice system, business, etc.
is biased against poor and uneducated people, which disproportionately affects certain minorities, but this is because these groups are disproportionately poor and uneducated, due for example to the previously-mentioned factors. Poor and uneducated whites are also treated poorly by the justice system.
Blaming the racial inequality on "structural racism" is counter-productive: First, modern racism is IMO not actually a huge part of what's causing this inequality, so you're aiming at the wrong target. Second, a lot of conservatives see this stuff as you
calling them racist even when they're not, which prevents you from ever getting their support. Third, policies which are designed to help one race over another (such as racial affirmative action or diversity quotas) in order to balance inequalities are themselves racist and unfair, and a lot of people despise such policies. Instead of complaining about "structural racism", it'd be more effective to support programs that help poor and uneducated people across the board. Since minorities are disproportionately poor and uneducated, any such programs will disproportionately help minorities; it's like affirmative action, but without pissing people off.
Many of the mainstream-left proposals are also simply ineffective: Affirmative action and diversity initiatives help a few individuals, but have little effect on the group as a whole. Many types of welfare encourage dependence on the government and discourage work, just making things worse. Government education sucks in general, and throwing more money at it rarely helps. IMO the solution probably has to do with ensuring stable family lives and good education. Ending the drug war and helping people addicted to drugs would be a good idea. Helping people relocate from inner cities without any good jobs to lower-population-density areas with decent jobs would be a good idea. Ensuring that children are going to good schools, have good nutrition, and are not distracted by home issues would be a good idea. (I'd ideally advocate doing all this through non-profits, not government.)