Why do we still have to read the work of dead white men?

There are a lot of reasons contributing to this, but the main reason is, perhaps unsurprisingly, racism and sexism. What we read in schools and universities is largely taken from what we call the literary canon, a collection of literature decided upon by academics throughout history. Historically, academia has largely been dominated by white men, and it makes sense that these men would have focused on literature that represented them.
Additionally, we can’t forget that women and people of color are constantly fighting to be recognized and taken seriously in the arts, literature included. In the past, women and and people of color's education was less focused on, as were their creative outputs. This lead to a cycle that let the canon be largely dominated by white men.
On the upside, we can see things slowly starting to change. While a lot of high-school and earlier education still focuses on a whitewashed canon, university-level English courses are seeming to make an effort to diversify their syllabi, something that could have a positive impact on the diversity of the literary canon in the future.