Honestly, because they change so fast and I feel not only do I get to know them but they need to get to know me. Each of my dogs has different motivators and different things they truly enjoy. They need to explore their world so grass is no big deal when they hit the rings - by the time they start showing in obedience they know that if a bird has turded, well they will get a bigger reward for not letting their nose go down. Because they have been worked through it and sometimes, gross as it sounds, released for some fun times rolling if it was a hard enough distraction, but they need to do their work/place first. They need to know I will always come back for them before I start leaving them on stays and I need to know when they need calm reassurance rather than walking in when they get anxious.
Because their bodies are changing so fast, it doesn't always make sense to me to start actual heel or obstacle training. For instance, I started training Casey young for the teeter and he did really, really great - until his center of gravity shifted around 8 months old and I did not have a strong tug drive going to get him through the pivot point. Lesson learned for me. To begin teaching heel they would need to look at my knees or calves at 10 weeks, but their focus will be much higher as adults - so I could use a target stick and move it higher as they grow but it seems just as easy to wait until they are almost at full height. Then they watch my hips and shoulders in obedience, agility and field.
I need toy, play, praise & food as rewards and this takes time to build. I need to get to know when they are confused, scared or uncertain versus just 'not doing that - no way no how' so I know how to work through the problem - this takes me time. How much pressure (not neccessarily corrections but if I move towards them, how close before they start fidgeting would be an early indication or their sensitivity to pressure).
And mostly because I love watching them grow and there is just so much to teach them that I feel overwhelmed if I start everything at once. This way they have say the hand touch - which I can use for set ups, heeling - they know targeting which I can use later for contact end behaviour, go-outs, broad jumps etc. They know left & right which I can use later in agility, directed jumping etc. These can all be used as release as stress relief too. They know a collar grab can lead to great fun in addition to just being a restraint.
And I guess one of the final reasons is, although I generally do not like comparing dogs to kids, just as a kid deserves to learn how to love life, play, be happy, where to turn for reassurance, love & safety as well as take constructive cristicism as needed, so does a dog. It is very hard to teach adult dogs to trust and play, but remarkably easy with most pups
ETA: I should also mention that I am happy with scores in the mid 190's; 198 - 198 1/2 has me feeling like a part of a successful team. However, an OTCh is not a goal of mine - I burned out with my first dog and realized the day I left the ring unhappy because we 'only' got a 197 1/2 and no placements that rather than striving for perfection I needed to shift my focus so I could keep the joy if that makes sense. I am a competitive person but obedience is a team sport; in order to continue enjoying my dogs I needed to step down from that goal - so I may not be the person to look to for how to start a competitive puppy out.