One reason they do this is usually around control. A lot of people feel like they have to be in control of everything in order to be safe. There may be ways you can make them feel safe without needing them to be in control.
Another reason is that until the thing is built, there's nothing to do, so everyone gets involved in that. This happens a lot in startups where they have a "build it and they will come" attitude. It's not true; building a sales/marketing channel doesn't actually need a product at the start, and there's a ton of stuff that needs doing while the product is being built. But it can be difficult explaining this to new founders.
Good luck :)