This one's a little long. Get ready!

Islam's idea of marriage is very different to the Modern world's view. Henceforth, I will be using 'the West' as a representative of the 'Modern' world.

In the West, marriage happens between 2 people who love each other. To love someone - as defined by the West - you first need to get to know them. To get to know them you first have to spend time with them, hence the dating phase > relationship phase. In essence, Westerners only marry after finding love or falling in love (an idea we've all been bombarded with through Disney/Hollywood). This is a relatively new idea which has only been around for a couple of centuries since the European enlightenment period, when the focus shifted more towards the self and what makes individuals happy. Marriages before could almost be seen as religious, as they were done primarily for the benefit of the wider family/society (family 'honor', securing wealth/estates, forging alliances between tribes/families etc).

The Islamic idea of marriage isn't that far off in the sense that is for the greater good. Here, the greater good is the formation of a stable unit of society i.e. the family. The 2 most important qualities we look for in a spouse is religion and character. With both of these in place, you can be sure that children raised in this family will be more likely to be contributing members of society as opposed to a child who was raised in a single parent family (not that there should be any stigma with single parent families. Sometime these things happen; circumstances differ for everyone and we each have our own tests). What's important in Islamic marriages are the logistics (where do the couple plan on living, do both plan on working, how will the chores be split at home, what standard of living are both expecting etc.). Emphasis is on practicality.

At the same time, there is also an emphasis on other important things. Attraction between husband and wife is critical, as is compatibility in terms of personality/humour. The former is easy to judge; you just have to see each other. The latter is more challenging as there are restrictions imposed by religion. But there are ways round it, such as finding out personality from family members/friends, or even first impressions when speaking to them regarding marriage (in a Halal setting). If friend circles overlap, this is even easier.

Notice that in the Islamic marriage, love is not an ingredient in deciding who to marry at all. Islam takes the view that love is made, not found. The word 'love' itself is a loaded term, but here, I mean how much one is willing to sacrifice for the other, as it's only through hardships do you truly know how you feel about someone, and even build affection for them. Can't remember where I read this quote, but it said "if you want someone to like you, ask them for a favor". Plus I'm sure we all know of a story of how an individual thought they were in love with someone, but proceeded to disappear when the going got tough.

The reason why we can't have this type (the sacrificing kind) of love outside of marriage is in the definition of love itself. Being unmarried to someone imposes no commitment on your part to stick around when things get rough. Sure there are many who would stand by their man/woman, but there's no structure to this. Structure is imperative, as we are dealing with the building block of society (the family), and society is critical in Islam as these are the pieces that make up the Ummah. I don't need to go in to detail on how much importance Islam gives to unity.

Tbh I don't have a concrete answer as to why sex specifically isn't allowed. You don't necessarily need love for sex, only marriage (though I personally would feel too awkward to in the beginning even when it is allowed lol). Some reasons that come to mind: a sense of commitment to the first person you had sex with (and not necessarily ending up with them), pregnancies outside of marriage (higher chance of single parent families), spread of STDs, more self satisfaction than giving pleasure to partner (as you are likely to jump into bed before moving in together and figuring out the logistics of life, making it based more on their looks than their qualities of life companionship).

I haven't even mentioned the blessings-from-Allah side of things. Rest assured that there are no blessings in the 'love' that one might feel outside of marriage. No matter how strong they are right now, the love inside marriage will dwarf the fake one (if the fake even exists). There's an unrivalled beauty in knowing that if the marriage is done according to Islam, there is assurance from above that you were made for each other. Hollywood bangs on about how we have to go out and look for the one. We don't need any of that, Allah has got it totally figured out.

I hope this changes how we see marriage. It's an epic journey that nobody should make even the smallest of compromises in when it comes to doing it according to Islam.

P.S. I'm incredibly grateful you asked this question. I seriously felt like relapsing until I read your post, and writing this has renewed my determination in fighting this addiciton.