salaam and welcome to the forum
Those who do haram and continue to go down that road are given leeway to continue that path. They want to live the kuffar life then Allah lets them live it. When death approaches then Allah will call to account for all that they did. This world has zero value before Allah so those who chase after it, Allah lets them have it.
Sahl bin Sa’d narrated that the Messenger of Allah (sal Allahu alaihi wa sallam) said: “If the world to Allah were equal to a mosquito’s wing, then He would not allow the disbeliever to have a sip of water from it.”
[ Tirmidhi Book of Zuhd , Classed as Hasan]
They may get married and appear to be happy and all, does not mean they will remain happily married for rest of their lives. There is no blessing in something based off of haram. Not only the major sins (zina, drinking) are an easy ticket to hellfire but their marriage is invalid as well if not done right and therefore they are now living a life of zina while under the assumption of being married.
If this zina took place before the marriage contract was done, the marriage contract is not valid unless it was done after establishing that there was no pregnancy by waiting for one menstrual cycle, according to the more correct opinion.
Marriage to a zaaniyah (a woman who has committed fornication) is haraam until she repents, whether the one who committed zina with her is the bridegroom himself or someone else. This is the correct opinion beyond a doubt, and it is the opinion of a number of the earlier and later scholars.
https://islamqa.info/en/175536
As for the believer. They have eman in their heart even though they may slip and do haram but they still fear Allah and feel guilty about it. Out of love for His true believers, He protects them by distancing them from what is haram. See this hadith for example,
http://share.pho.to/AuDmk
Also, those who are righteous get paradise in the hereafter:
“Whoever works righteousness — whether male or female — while he (or she) is a true believer (of Islamic Monotheism) verily, to him We will give a good life (in this world with respect, contentment and lawful provision), and We shall pay them certainly a reward in proportion to the best of what they used to do (i.e. Paradise in the Hereafter)” [al-Nahl 16:97]
Those who are heedless, while they may seem happy , Allah gives them life of hardship and raises them up blind on Resurrection day.
“But whosoever turns away from My Reminder (i.e. neither believes in this Qur’aan nor acts on its teachings) verily, for him is a life of hardship, and We shall raise him up blind on the Day of Resurrection.” [Ta-Ha 20:124]
https://islamqa.info/en/12804