The reason for this is chain of custody. They should be able to prove that from the time that the person last had the device until the point when the evidence was collected, no one modified it. And from the point where the evidence was collected, until when its presented to the court, no one modified it.

But these type of vulnerabilities present a problem, in that reading the device could/would modify timestaps of the data captured. The solution is to not use Cellebrite, there are lots of forensic analysis tools. To be effective Signal would need to exploit the major vendors equally.