Because the powers that be and the systems they have in place (capitalism, Christian white supremacy, patriarchy, cis-heteronormativity) benefit in one way or another.

If they teach us that Julius Caesar was a bad guy and that it's good he was defeated, then we might learn that our current leaders are often bad guys too, and that maybe we should do the same to them.

In the same way that if they teach us that Hitler took his inspiration for the holocaust from already firmly established American racism, we might learn that our own history is just as bad and should be fought against at all cost (which is also what we're taught instead of the reality - the allies fought the Nazis because they threatened their own power, not because of an ideological disagreement).

That's why we're not taught (or only given a palatable token example) about working people fighting the owning class for basic rights, Black brown and Indigenous people fighting the Christian white supremacist establishment and winning, and other oppressed groups standing up to their oppressors (E: nor most of the atrocities they have and continue to commit).

Whitewashing history is always a deliberate act, and is always done in defence of the ruling class.