Because both the government and corporations don't want to be giving you plus your 7 wives/husbands benefits.
You shouldn't be limited to one partner in terms of love/sex/living together/whatnot if you so desire. Why the hell should I care? But let's say that your job offers healthcare for you and your partner - if you have 18 partners, that's a hell of a different cost from one partner.
Likewise, let's say that you slip into a comma. Who has say over that feeding tube? That's already complicated! Let's say I'm some rich jerk trying to skirt the inheritance tax. Well, spouses are one way around it. Well, clearly my kids are also my spouses! Who gets to visit you at the hospital and who doesn't? Well, spouse 1 says your friend Joe should come, but spouse 2 hates Joe. Does it work by majority voting? What if they can't find all your spouses and you're dying and Joe really wants to see you?
So, if you think of marriage as the government telling you that you can only love/have sex with/live with one person, it seems oppressive if you desire something else. If you see marriage as this standard government contract that says you are allowed to designate one other individual for partner benefits, hospital visitation, inheritance, etc. then it makes sense.
The problem is that our society equates marriage with love/sex. Marriage should probably be seen as a contract. As such, you can see how it wouldn't really work to designate 6 people as having the right to determine whether you stay on life support. Yes, you could simply create individual contracts specifying everything that marriage entails, but that's tedious, expensive, prone to error and requires everyone who deals with you to have a lawyer to interpret that agreement - and they're not going to want to spend hundreds of dollars for a lawyer and tons more in liability to interpret your individualized contracts so they'll just deny you. Marriage is a standard convenience contract and shouldn't be confused with love or sex.
//I'm really interested to see if you disagree with this or how you think things like benefits/hospital visitation/whatnot should work in poly relationships (yeah, sociology student ;-))