A reason is literally an excuse.

Not by default, though i will say i phrased my reply poorly, what i should have said was.

a reason isn't always an excuse

They are similar, but the implications and potential usages of each are different.

A reason is an potential explanation for an action/outcome etc.

An excuse is an attempt to use a reason to mitigate some or all of the consequences of an action.

An excuse always has a reason as a basis, but not all reasons are excuses.

The main difference is intent,

As an example, two kids get in to a fight, kid a is asked "why were you fighting?"

They say "they hit me so i hit them back".

That can be both a reason and an excuse depending on the intent of the speaker.

As a statement of subjective fact, that would be a reason.

As an attempt to dodge punishment or consequences, it would be an excuse.

It can also be both.

If every time your parents offered you ice cream as a child, they beat the hell out of you, you’d soon start associating ice cream with with abuse. You’d expect it, as a matter of survival.

When you got older, someone offering you ice cream would trigger that fight or flight mechanism.

While potentially true, I'm not sure how it's relevant.

Mitigation is quite literally freedom from one consequence, in place of another.

That's not what mitigation means either.

All excuses are reasons

Not all reasons are excuses

Feel free to send me a link to a definition that states reason and excuse are synonymous backwards and forwards.

I'll wait.