Because I think you're assuming a downward sloping demand curve, which doesn't hold.
Here, think about this:
We've hit the zero lower bound. The FFR is down to zero and so is the yield on 3 month T-bills. If the Fed suddenly buys half of all the 3 month T-bills, does that push their yield further down? Can the Fed lower 3 month T-bill rates as negative as they like just by purchasing more and more?
Yes, we've had brief periods when t-bills yielded less than zero. But the 10 year isn't at zero.
Right, so we agree that there isn't a downward sloping demand curve for 3 month T-bills? The yield on them can't be lowered by the Fed buying more?
Halfway there. Demand curves don't always slope downwards. Here the demand curve is horizontal.
Now what determines the yield on longer Treasuries? The sum of expected short-term rates (the term structure) plus risk premia (default risk and liquidity risk are around zero anyway for Treasuries) plus term premia.
So when the FFR isn't at the ZLB, lowering it changes the term structure, and so changes yields all along the curve. When the FFR
is at the ZLB, it can't be lowered any further since the demand for short term T bills becomes horizontal, and demand becomes horizontal all along the yield curve too.
So buying long T-securities (without changing forward guidance) doesn't lower their yield because what have you changed? You haven't changed the term structure. You haven't changed risk premia. You haven't changed term premia. You've changed none of the things which determine the yield on a long bond.
The only way to change the yield is to guide expectations about the future path of the fed funds rate, changing the term structure.