Why does the simplicity of the answer not seem to be enough?
Because some people desire to be tethered and bound to this world by their own work and their own struggle. Somehow they don't seem to really, truly trust God, and so they have to add to what He did on the cross.
As if they ever could.
I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain." -- Galatians 2:20-21"I am crucified with Christ: neverthless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me.
“Somehow they don’t seem to really, truly trust God, and so they have to add to what He did on the cross.”
...and do you trust these words from Jesus?
John 6:53-58, 66-67
“So Jesus said to them, ‘Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is food indeed, and my blood is drink indeed. He who eats my flesh and drinks my blood abides in me and I in him. As the living Father sent me, and I live because of the Father, so he who eats me will live because of me. This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live forever.”
I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain." -- Galatians 2:20-21
It is so clear "believe", just believe everything else will follow. The key to the kingdom is the indwelling Holy Spirit which is in in all believers.
We preach Christ Crucified and we are called foolish or lacking sophistication on these threads. You've seen it happen several times. It's even more sad than putting flowers on the feet of a statue on Resurrection Sunday.