Because. In Elite: Dangerous, it's Dangerous and you are Dangerously stuck with your mistakes FOREVER!
My first system is a total disaster, I just started building stuff - like ooh, that looks cool and shiny! Oooooh! That looks cooler and shinier!. Then realized it was all wrong. Then realized there were no takebacks. Then said F*!K it and just went off building then most nonsense system ever. And it's turning out to be pretty excellent. So, build what you want, make mistakes and learn for the next system.
I mean, you already have 15 systems bookmarked right??? Or is that just me?