I guess your argument hinges on the fact that the length of days are equal at equal distances North and South. You're making the assertion, remember, not the Flat Earthers. If the Flat Earthers don't believe your assertion is true (which I don't), and if you don't offer scientific proof of it being true, of course no Flat Earther is going to bother answering. That's almost like asking a Creationist "If creation is true, why do animals evolve?"
Burden of proof and all.