Unfortunately a lot of investigations start out as speculation/vibes before they turn into an actual evaluation. And getting past speculation/vibes can take a lot of effort and political/social/professional capital before even starting.

Well yeah. If they had solid evidence at the start, why would they need an investigation?

It's not as obvious of an answer as it initially sounds. Coming at this from a stint in investigative journalism where even beginning an investigation requires getting grants and grants involve convincing other people that the money is going to good use. Also having been told that an investigation I ran was nothing by multiple editors that turned out to be something big... it really shifted how I perceive investigations and what it means to stick your neck out when everyone's telling you that something isn't happening even when it is.

Vibes are a perfectly solid ground to refuse to engage with something.