Elizabeth risked political suicide if she returned Drake's £400,000 windfall, as the monarchy's survival depended on raising money.
The £400,000 Drake brought back was larger than Elizabeth's annual income, making its return impossible for a monarchy chronically short of funds.
transcript
Tom Holland: So, remember you said 400,000 was bigger than her entire annual income, right? Yeah. I mean, this is a point at which the great issue for the monarchy in the 16th and 17th centuries is raising money. So, for her, a windfall like this, it would be suicidal. It would be insane to give that away. You're always going to keep it.