Why can video footage sometimes not be used as evidence in court?

Information that was not collected in accordance with the law is not legally usable. A dashcam video, for example, may only be used to document the driver's own behaviour, not to prove someone else's offence.

"A picture is worth a thousand words" does not always hold in court, because legal requirements restrict what can be done with recordings. In a Zurich case, a dashcam video of other road users was not admitted as evidence.

The ethical point: the end (catching an offender) does not automatically justify the means (unlawful recording). The rule that unlawfully obtained evidence cannot be used protects everyone from being filmed without a legal basis.

Go deeper:

• 20 Minuten: Dashcam-Video nicht als Beweis zulässig — the Zurich dashcam case.
• Wikipedia: Fruit of the poisonous tree — the general idea of excluding unlawfully obtained evidence.