Why do farmers need short hedge?
"Why can't farmers simply sell their wheat ahead of time to buyers who are capable of storing this wheat..."

"...sell their wheat ahead of time..." is precisely what a futures contract does.

To imply that it is necessarily speculators on the other side is misleading. 
On a futures exchange, trading is anonymous and identity of counterparty is not known. The counterparty can be a hedge fund taking a long speculative position or a cereal company who intends to take physical delivery. 

For the short hedger (farmers in your example), it doesn't matter who is on the other side of the hedge. The futures position eliminates price risk regardless. It's a risk sharing arrangement. The party which has more incentive to shed risk must induce the other side to take the trade. If the short side is more risk averse, this leads to backwardation. Otherwise, contango.

(In particular, empirically expectation hypothesis does not hold for physical commodities. For financial products, no arbitrage would tell you EH holds not under the physical measure but the risk neutral measure.)