My response, and this is only my opinion....

it is to reduce risk.  If a farmer plants seed, and the seed does not come up, the farmer goes back to the seed company and says "your seed is bad. I want a replant."  By using requiring seed treatments, the seed companies reduce the risk - their risk - of having acres that need to be replanted.  The farmer pays for the seed treatment - in the cost of the seed - and the seed company reduces the money it lays out in having to provide seed for a replant.

That is my take on it.