My impression is that the investigation is changing over time. First police were told that she didn't show up for brunch with friends. Friends were concerned that she was missing, so they contacted her husband and he phoned in a missing person report.That we, the public, know of. If police are certain she left, but don't know what direction, the footage must determine that she did in fact leave. They didn't waste days searching bushland for funsies. We are just not privy to seeing it at this stage.
Police found CCTV of a woman running, believed it was her - did someone confirm that, or did police just assume it was her without input from anyone who new her? For a few days, police believed that had evidence that she went running.
Then police learned from a 3rd party that the person running is not Samantha. Police then started looking for CCTV between her house and the park. Then they suspended the park search because they wanted to review CCTV footage. I'm guessing that they have no CCTV footage of her between her house and the park that morning.
When there was confirmation that she went running, it was not suspicious - got lost, dehydrated, etc. Without that confirmation, it's somewhat suspicious.