Because that is what we all deserve, and will be found guilty in the judgment
Paul is not talking about what we “deserve” - he is talking about what we will GET. In Romans 6:15, Paul asks, “What then? Are we (believers) to sin because we are not under law but under grace? He answers this with “By no means!”, and warns believers that sin leads to “death”, while righteousness (ie, keeping the commandments) leads to “eternal life”. He doesn’t say, “Oh, you believers don’t have to worry about sin … you will be saved regardless of your sins”, which is what you seem to be claiming.
In Gal 5, Paul clearly tells believers that their sins can mean they “will not inherit the kingdom of God”. Which part of “will not inherit the kingdom of God” sounds like salvation? Paul is here preaching the same dire warning that he preaches in Romans 6 - that the sins of believers can lead to eternal death.
Here are three other verses in which Paul preaches that the sins of believers can lead to eternal death:
”He who believes in the Son has eternal life; he who does not obey the Son shall not see life, but the wrath of God rests upon him” (John 3:36);
“Strive for … the holiness without which no one will see the Lord” (Heb 12:14);
“He who through faith is righteous shall live” (Romans 1:17).
Jesus Himself preached the same warning as Paul - in Matt 7:21-23 he condemns certain believers to Hell because of their “lawlessness” - aka sin.
You don’t preach what Jesus and Paul preached.