Why are more people into casual dating and hook-ups as opposed to actual relationships?

I wouldn't say that "more people" are into this. I'd say that more MEN are into casual dating and hook-ups and because that's what most men want, many women are settling for that. A lot of women go along with this type of arrangement in the hopes that it "turns into something" after a while, which it almost inevitably does not and the women is left disillusioned and angry at men as a general populace, stating that "men are pigs", etc.

My personal belief about this (whether we're talking about men or women) is that people are DEATHLY afraid of being vulnerable. Most of us want a boyfriend or girlfriend when we first start noticing the opposite sex. But somewhere along the lines that can change if we've been hurt. Maybe the girl we really liked ridiculed us, or we watched her go out with someone else and had our heart broken. That can lead a lot of people to vow "never again" and so they close themselves off from anything serious or anything involving an emotional connection because they just don't want to get hurt. It's about having all the fun without any of the risk.

A COMMITTED relationship with ONE PERSON for A LONG TIME gets Boring.
No one says it, but they all feel it. Long time BFs and Husbands Do Not tell their women they are "sexy" or "beautiful" anymore. They don't treat each other with Value, Attention or even Respect.
When you Desire someone but they Are Not Yours, you treat them special and with more care so you don't lose them.

Besides, why have ONE Guy or Girl, when you can PLAY for any Guy or Girl... and never be bored.

Being in a relationship is not for everyone. I don't know why some people are so hung up on what others are choosing to do with their lives. What works for you may not work for someone else.