Because many of the rules (minimum wage, age restrictions etc) do not apply to farm businesses.
I guess it made sense when 80% of the population were farmers, and family farms involved 6 sheep that the 8 year old children liked to help milk...
But in today's world of big-business farming, separating it out, either for stats or labor laws, makes no sense IMO.
Milking sheep?
Yes, you can buy sheep milk and cheese
You can milk anything with nipples
Is there some rationale for this? I'm surprised that farms specifically get a pass on things like that.
It's just tech debt
Perhaps partly because farm business is seasonal in ways that non-farm business is not?
Note that nonfarm payroll is still seasonally adjusted - 156105k vs. 156306k adjusted.
The FRED series for NFP with and without sesonality: https://fred.stlouisfed.org/graph/?g=15LPE
Farm business relies either on family or very low wage immigrant labor. They behave differently enough that it's worth watching them separately.