HPATOOTP - Why didn't they use something like Dumbledores pensieve to just show Harry's memory and prove Voldemort was back?

Just watching it now, back to back to the previous film and it struck me, surely that'd be define proof? I've only watched the films, sorry if it's an ignorant question.

Edit: everyone's reminding me that memories can be altered, which i complelty forgot about, makes sense now! Thanks 😁