Which does have its unfortunate aspects. Just 'cause it's read from front to back doesn't mean it's built that way.
If a story is built up from moments of inspiration, there's no rule that the moment that inspires the story has to be the opening. It can be, if it doesn't matter, but it can also be the climax, the middle, even the denouement. The first stage of writing can be a welter of disorganized notes.
Even once it's going, the opening can often be revised, and I've had to jump back to fill in the supporting structure for a new plot twist more often. An opening can prove to start too soon, or too late -- there's a reason why musicals have the proverb that you write the opening number last.
Still, writing from an opening may require it to be strong enough to support at least a first draft. It doesn't always work to not get it right, get it written, because it's steering you down false paths. . . even if it takes five or six drafts to find one that won't.