Launches get scrubbed all the time. You want that to happen when they find a safety issue with a human space flight rated rocket you know.

Heck, half the time, non-human spaceflight launches get scrubbed. I almost never see missions I work on or around get off the ground in the first launch window they get. If they do, it's pure coincidence that all of the safety checks, weather, etc all lined up at the same time for a safe launch.

Thing is, these things are billion dollar projects, even without humans in it, you don't want it blowing up on the launch pad, or having a stuck valve, reaction wheel wig out, or anything else go wrong.

The amount of stuff that has to be right on a rocket is staggering. Makes the safety checks for planes look like childsplay...why? Bcz you're launching a guided bomb essentially and trying to keep it from detonating. That's not easy to do.

Edit: For anybody that wants to retort that SpaceX did the same thing with less money. SpaceX was extremely reckless, and blows up their rockets all the time, they spin out of control, lose control and pitch toward the Ground. (And that last one is recent).

They assume a lot of risk, and their management is fine with it. Wasn't until they put humans in them that they had to get serious and follow all safety stuff.

Nasa is also very much involved in this one, and gets a lot more blowback when a launch doesn't go well, so they are cautious about it. Rightly so.