How come I can't put the closing cost to the back of my mortgage?

I am buying a new construction home so I have already paid $500 for the earnest money. Then I have to pay fha for $5900. I heard my seller is going to pay $3000+ taxes but I also heard from my lender I will have to come up with the rest of my closing minus the earnest that will be about $7700. My lender said I cannot put the closing to the back of my mortgage. Does that even sound right? I have enough for the down payment but not sure about part of the closing as well. I am coming up with this very quickly.

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