It is far from a known event that Seetha was not born to a mother, rather was unearthed when the king Janak was ploughing the earth.
In my understanding the ploughing is associated with farmers and why the king was on to that action?
Janaka was not ploughing for farming purposes, he was doing the ceremonial ploughing performed in the beginning of a Yagna, as described in this chapter of the Bala Kanda of the Ramayana:
Later, when I was ploughing the ritual field then raised by the plough [from the furrow is a baby girl... since she is] gained while consecrating the ritual-field, she is named as Seetha, and thus she is renowned. Hers is a non-uterine birth as she surfaced from the surface of the earth, but fostered as my own soul-born girl and I determined [to give her in marriage to a bridegroom where his] boldness is the only bounty.
By the way, this is the same way that Venkateshwara's wife Padmavathi was born; the king Akasa Raja was doing the ceremonial ploughing for a Putrakameshthi Yagna when he spotted a baby girl. See the Tirumala Sthala Purana I linked to in my question here.
Society at the time of Janaka was an agrarian society. Kings led by example. Janaka's name means 'Progenitor' or 'Father'. Sita means 'Furrow'. A plow is used to place the seed in the earth and 'create' the furrow. By plowing the earth, Janaka - the father - was placing the seed into mother Earth. Sita was not born of a human mother, but mother Earth. The story is highly symbolic, although not having a human mother, Sita is still born from the union of a father and mother. See Ramayana 1.66.14-15