High Frequency Transformer - Efficiency
It is basically an economic question, you can get almost arbitrarily close to 100%, the price just rises exponentially, massive cores at very low flux density, wound with superconducting cables, in liquid helium? Sure you could do it, but it makes no sense.

there is also little point in making the transformer very much better efficiency wise then the rest of the circuit, if you are overall say 95% efficient, so 500W or so of waste heat, but the transformer is 99% efficient (so it contributes 100W of that 500W), then making the transformer 99.9% efficient will leave your total power losses at 410W, getting the drive electronics efficiency up would pay bigger dividends. That last 0.9% has exploded the cost of that transformer, probably by at least 1000%.