Experience is not necessarily a big factor, and what qualifies as "experience" is highly dependent on context. However, the experienced person generally has just seen more than the inexperienced person. They've had a chance to deal with stuff the inexperienced person just hasn't. That counts for something; in some (most?) cases, quite a lot. It's a bit silly to dismiss experience out of hand, or to minimize the role it plays.
Also, "languages and libraries and frameworks" don't really change all that often in practice, in most places where software has been employed as a solution. That's a very "Web Dev is All Software Dev" (read: myopic) Valley-centric belief. You're referring to very narrow, niche instances where startups "pivot" frequently (or can't make up their mind, or are run by fad-chasing hipsters).