# Why the universal Turing machine simulation in O(TlogT) cannot be applied to transform multi-taped Turing machine into single-taped?

Recently, I've read Hennie's Paper. I understood the construction of buffer zones, but why can't it be applied to yield a single-taped Turing machine?

Just because that UTM itself requires $$2$$ tapes to do its task that efficient. Removing one tape, the magic is gone.
But assuming you have the fantasy power to do efficient $$1$$-tape simulation, what gains do we have then?