Because it's not like there's just ONE centuries old tradition. I mean, hell, my great-great grandfather showed up on a farm in Kentucky, started working for the family, then "married" the Farmer's daughter simply by agreeing to do so with her father. This would have been in the late 1910s to 1920s.

Then there are other centuries old traditions, like requiring the bride's family to pay the groom a dowry... or the centuries old traditions of requiring the groom to pay the bride's family for the bride. Or what about the centuries old traditions that say you can marry as many wives as you can fiscally support? Hebrew law allowed for that. So long as you could financially support, and pay them enough attention each, you were allowed to have as many wives as you wanted.

Even in the early church it was acknowledged that many men had more than one wife. Only leadership was limited to having a single wife. It was one of the qualifications of being a Bishop, that a man be sober and temperate and husband to one wife.

All of that ramble to say "centuries old traditions" are not enough excuse to limit someone else's freedom. Nor are they excuse enough to keep doing something. Just because a thing has been done a certain way for a long period of time does not mean it shouldn't change.