What is the purpose of marriage?
To establish the church in one's household just as God created Adam & Eve.
Why should we choose to marry?
Because we were not meant to live alone except for monastics who devote themselves to God.
Why have I not seen threads here about people discerning marriage vs. being monks and nuns?
People have discussed marriage and monasticism on this board. As for discerning, you're still responsible for your own life without needing an Internet forum to tell you which direction to go (or not to go). Plus, a lot of people don't like talking about themselves on an Internet forum whether they're interested in marriage or monasticism....