Because if you had the chance to do it again, having already done it once, you'd do it somewhat (or entirely) differently the next time. Nearly always.
Ever build anything fairly big?
Like restore a car, install windows in your house, tear out walls, move electrical wires around, pour a concrete patio, build a kitchen, put on a roof, or anything like that? I've done all of these things, and no matter how much research I did up front, after actually experiencing the process, there are always some things I would do a little different to save time, money or simply make things easier if I were to do it again. It's human nature and I think it's perfectly normal. Then again, if I were to do it all over, I'd probably learn even more on the second generation and still do it again somewhat differently the third time.
If you have done any projects, this should not be a mystery whatsoever. If you've done something so much that perfection is the norm, you've been doing it so long you forgot the learning curve you went through. I went through something like this before I left my first position working in the fab. When you can know your schedule down to every darn minute and know that you are doing it the best way it can possibly be done because you've tried every other iteration possible, it's time to move on.
I met an anesthesiologist in college while working in the hospital that had a very successful practice, made oodles of money, and still always said. If I had to do it over, I'd do it all differently simply because of what I've learned doing it this way.
