Because they don't know how to "walk in the Spirit."
Galatians 5:16
16 But I say, walk by the Spirit, and you will not carry out the desire of the flesh.
Because they aren't aware that there is a difference between "living in/by Spirit" (being saved) and "walking in/by the Spirit" (being led of the Spirit).
Galatians 5:25
25 If we live by the Spirit, let us also walk by the Spirit.
What's the chief difference between "walking in/by the Spirit" and "living in/by the Spirit"? Paul explained:
Galatians 5:17-18
17 For the flesh sets its desire against the Spirit, and the Spirit against the flesh; for these are in opposition to one another, so that you may not do the things that you please.
18 But if you are led by the Spirit, you are not under the Law.
Romans 8:13-14
13 for if you are living according to the flesh, you must die; but if by the Spirit you are putting to death the deeds of the body, you will live.
14 For all who are being led by the Spirit of God, these are sons of God.
Most Christians are not being "led by the Spirit" throughout every day. They don't know what such living looks like because no one around them is living in a constant state of "being led" by him - not even their pastor(s). And since their spiritual leaders give only lip-service to life in the Spirit, knowing nothing, practically-speaking, about what it is to "walk in the Spirit" throughout every day, they don't ever teach those under their spiritual care about "walking in the Spirit." As a result, these ignorant believers do not "walk in/by the Spirit" and so cannot avoid carrying out the sinful desires of their flesh. They try their best in their own fleshly, human powers (intellect, willpower, physical strength) to please God, but life in the Spirit, the only life that truly pleases God, is a supernatural business and entirely beyond human capacities to properly achieve. But, since everyone else is doing the same, these poorly-taught believers just assume that a regular, tight cycle of sin>confession>sin>confession is normal and resign themselves to it.
Consider Paul's words, though:
Romans 6:1-2
1 What shall we say then? Are we to continue in sin so that grace may increase?
2 May it never be! How shall we who died to sin still live in it?
Romans 6:6-7
6 knowing this, that our old self was crucified with Him, in order that our body of sin might be done away with, so that we would no longer be slaves to sin;
7 for he who has died is freed from sin.
Romans 6:11-12
11 Even so consider yourselves to be dead to sin, but alive to God in Christ Jesus.
12 Therefore do not let sin reign in your mortal body so that you obey its lusts,
Galatians 5:24
24 Now those who belong to Christ Jesus have crucified the flesh with its passions and desires.
Galatians 6:14
14 But may it never be that I would boast, except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world.
Colossians 3:2-3
2 Set your mind on the things above, not on the things that are on earth.
3 For you have died and your life is hidden with Christ in God.
And so on.
The crucified life was a big deal to Paul. He directly and repeatedly connects practical sanctification of the believer to the truth of the believer's being crucified, buried and resurrected with Christ spiritually and thus set free from the power of the "old Self" and sin (
Ro. 6:6, 11; Ga. 2:20). When's the last time, though, that you heard a comprehensive sermon series on "walking by the Spirit," the crucified life, submission to the Holy Spirit and life in the Spirit (conviction, strengthening, illumination, comfort, transformation, etc.)? Very, very rarely (and never in a systematic manner) in my fifty years of walking with God. Baptists, in particular, seem deathly afraid to discuss "life in the Spirit," though it is a common (and vital) subject of the NT. They generally remain in over-reaction to the blasphemous nonsense of hyper-charismatics (Benny Hinn, Todd White, Paula White, Kenneth Copeland, Mike Bickles, etc.) and so have blinded themselves to a crucial aspect of Christian living.