Because when we (follow) the flesh we die quicker. However, when we (die) to flesh we live through Christ.
Hmm, that to me doesn't fit too well with the context.
Yes, and I will rejoice, for I know that through your prayers and the help of the Spirit of Jesus Christ this will turn out for my deliverance, as it is my eager expectation and hope that I will not be at all ashamed, but that with full courage now as always Christ will be honored in my body, whether by life or by death. For to me to live is Christ, and to die is gain. If I am to live in the flesh, that means fruitful labor for me. Yet which I shall choose I cannot tell. I am hard pressed between the two. My desire is to depart and be with Christ, for that is far better. But to remain in the flesh is more necessary on your account. Convinced of this, I know that I will remain and continue with you all, for your progress and joy in the faith, so that in me you may have ample cause to glory in Christ Jesus, because of my coming to you again. Philippians 1:18-26 (ESV)
Paul is writing this letter from Prison and is assuring the Philippians that he will not lose heart, but will stand up for the gospel so that Christ will be honored in his body whether it be honored by him continuing to live and striving for the gospel. Or in his martyrdom, where he sacrifices his life for the sake of the gospel. Then he uses the explanatory conjunction of "for" to Paul, to live is Christ, his life is all about knowing, magnifying and proclaiming the name of Christ. Yet, dying to Paul, would be gain.
He goes on to explain. He is torn between continuing to live, or dying. Why? His hearts desire is to depart and be with Christ, which explicitly denotes the fact that he would be with Christ upon dying (departing=dying), but he he knows that right now he must remain in the flesh for the sake of the Church.
To sum up the conclusions we can draw from this.
Dying for Paul = Being with Christ, which is far better.
Continuing to Live = Fruitful labor for the sake of the Church.
I don't see why one would want to argue against the idea that we go to immediately be with Christ upon dying. That is a blessed hope that I think we should all have.