I think the fundamental thing you're missing is that a gene's behavior throughout your life depends on more than just its sequence. Everything else that goes on in your body, and in particular everything that goes in and comes out of your body, can impact when genes are triggered to form proteins, how those proteins fold, and what effect the proteins have.

At best, gene sequence data like you describe could provide rough probabilities for some health conditions. That's as likely to lead to over-treatment for a non-problem as catching a real problem early.

Sure, there would be some cases where everyone who has a certain sequence winds up with the same disease, and it'd be good to discover those early in life. (Ex: my wife was born with Wilson's Disease, which causes liver failure by early adulthood 100% of the time without treatment.) But those cases would probably be very few compared to the number of people who get freaked out by things that might become an issue, but probably won't, and the cases where the correlation is too weak to provide a warning, but a health problem occurs.