But not everything has to be re-invented from scratch, assuming people aren't morons.
Some things do, even if people aren't morons. Just because somebody out there knows a really good way of doing something doesn't mean the people who WORK FOR YOU know how to do that. Intellectual capital isn't always completely accessible to a new organization, and sometimes you have to build your own rather than try to lure/buy/tempt it away from other organizations that are still using and/or protecting their own.
A good example of this is the development of SpaceX's exploration programs. While they did in fact draw on a lot of the scientific and engineering background developed by those who came before them, alot of the practical issues that make their rockets and capsules worked had to be developed by trial and error. Even when they finally managed to get experienced people working for them from the astronaut program and the broader aerospace industry, they STILL had to do some things from scratch, just because the applications of what they were using those techniques for were so radically different.
As Buzz Aldrin once put it, specifically about SpaceX: "They don't even know all the things they don't know." They literally had to bust their asses for years just to figure out questions they were supposed to be asking. They're still doing it now; it's a VERY long process.
What you said was true because people were devising a completely new system that had absolutely no precedent.
Exactly. Just like Starfleet, your assertions to the contrary. Earth had NEVER built an interstellar exploration force using starships capable of FTL travel. They were doing everything for the first time and had no idea what was going to work and what wasn't. The only way to find out was to try things and learn from experience.
You're planning on operating large vessels that will be away from their bases for extended periods of time with largish crews that allow constant mission activity when they're away. Hmm, what examples could there be for large ships away from their ports...with large crews...mission activity... Hey, guys, why don't we start on the basis of how a wet navy does it?
You're planning on operating large machines that will attack at high speed in support of far less mobile infantry. Hmmm... what examples could there be for large things that attack at high speed with more mobility than infantry... hey guys, why don't we start on the basis of how mounted cavalry does it?
It took the air corps TEN YEARS to figure out what a fucking stupid idea that was. I suspect it took Starfleet even longer.
Put it simply: starships aren't boats anymore than tactical fighters are horses. They're analogous in some ways (soldiers who ride in helicopters call themselves "air cavalry, even!") but they are not the same thing and don't use anything close to the same tactics.