How come none of these high-profile celebrities get pursued for drug charges?

I mean, just as an example, the case of Depp v Heard. There was so much evidence of cocaine, her admitting she took mushrooms etc. I get that it wasn't the point of the trial, but how come admitting to all this illegal activity and showing proof of it doesn't lead to a conviction? I mean, if it was just part of evidence for a trial, could you kill someone too?

Again, I have no legal background or knowledge, so I'm asking as a complete pleb.