There's a difference between gathering evidence for impeachment and using it for an impeachment
Many people are pushing for impeachment hearings to begin immediately, but one of the reasons the various committees need to continue their investigations is that the gathering of the evidence is often tedious and messy. Hearings in this regard can be very informative, but won't necessarily tell the whole story because the House will still be digging for information and it won't all be tied together. The hearings will be interesting, but they won't always be good television. This also needs to be done by committees other than Judiciary since they involve different subject matter that isn't within Judiciary's purview (for example, intelligence, financial services, etc.)
However, once more evidence is gathered, the pieces can be pulled together and presented for impeachment in a much more coherent and cohesive manner that tells the story and makes the case.
That's what happened in Watergate. By the time the Judiciary Committee took up impeachment, the public was much better educated about the depth, breadth and nature of Nixon's wrongdoing. It was less a matter of the Judiciary Committee getting in the weeds in an effort to gather evidence, but more of doing a deep dive into the large volume of evidence that had already been gathered and then shaping it into a clear narrative that showed the public that impeachment was the only just result.
That's one of the reasons I'm glad that Pelosi and other House leadership are resisting the pressure to jump into impeachment hearings now before the various committees have done their investigations. They know that will help ensure that impeachment will be based on a solid foundation.