First of all, know that any reasoning given about Allah's rulings is only an educated guess and not a definitive answer unless Allah clearly gives an answer. The only sure statement we can make regarding this is that Allah forbade it so humans are not allowed to do it.
That being said, the possible reason for this being haram is the same as that of adultery. It ruins the family unit.
It cheapens what is meant to be gotten through marriage alone. Allah created men and women so they could be partners to one another. Giving intimacy outside that partnership means that people value the partnership less.
For a simple demonstration of this, look at the countries where this deed has become widespread. You will notice the following things:
Age of marriage is higher, meaning people spend less time in the partnership (marriage) Allah created for them. This happens because people know they can get intimacy in casual relationships and uncommitted ones.
Divorce rate is higher and remarriage is low. Again, this happens because people know they can get intimacy without marriage.
Rate of unmarried people is higher. Same reason.
All this combined creates a society that is uncommitted with regards to marriage. This is completely morally wrong, and causes among many other effects, low birth rates, people developing less responsibility, and people thinking more about inappropriate things in general.
And this is not even getting into the psychological problems. If people lead a promiscuous lifestyle in their developing years (teens and twenties), that leads to their brain getting used to it physically and psychologically. This means they will forever be stuck with such a mindset even if they are able to inhibit and control it. This causes nothing but problems in the marriage later on if one person is unable to control their desires.
And it is not even as if one can claim that the people who will get adverse effects will not take part in it. Opening the door to this causes almost every person in that society to be drowned in these problems. The culture itself changes to encourage it. I will let you look up the statistics of how massively prevalent it becomes in a short amount of time yourself.