Because endorsing sin while pretending to profess salvation is worse than no church at all.
Precisely. Such a church preaches a false gospel. God’s word tells us to be aware of, and avoid false preachers who come disguised as God’s elect. Folks that continue in such a church are being deprived of the true word of God. THey shoukd find a church that doesn’t compromise with the Devil.
If you truly loved your brother would you lie to him and cover for sin?
In doing so you will lead him off of the true path of salvation.