My original (for which I'm being burned at stake by the site):
Historians don't? History is said to be written by the victors and you can't very well write up on your achievements if you have already departed the mortal plane.
For example, we will never really know just what kind of king Richard III of England was due to Shakespeare's famous plays (written during Tudor times) distorting our views. While Richard III is not exactly going to come back himself to tell us either.
Lets try again:
'History' that is assessed by historians relates to past events that may have happened a long time ago - too long perhaps to be able to question anyone alive who was present at that time.
If this is the case, historians would have to rely on a variety of primary sources (e.g. original treaty document that ended a war) and secondary sources (e.g. someone not involved in the event describes the event in a book perhaps several decades) that describe that event.
If there are sources that describe both sides / multiple perspectives of the event, then historians can try to piece together an understanding of what really happened.
If sources completely oppose each other then this can be tricky. Case in point - the Egyptian–Hittite peace treaty, the treaty document is written both in the Hittite language and in the Egyptian language.
However, both accounts provide opposing details to the outcome (both sides claim victory in the same battle).
In this case, the bias involved in the sources can be dealt with in the sense that we know that at least one or more source(s) is/are not completely trustworthy.
However, we do not know the real outcome of the battle - both factions are not going to tell us what really happened.
So, perhaps, in the sense that we cannot piece together the truth, I referred to historians not being able to deal with historical bias. The sole meaning of the last sentence is intended to reflect the fact that the bias cannot be overcome to find the truth.
[can I please get more constructive feedback on this (not just a simple -1, but a real comment like Sardathrion's)]