This is necessary because there is no way for a judge to ask a deceased person, if the will presented in court accurately describes how they wanted to distribute their estate.
Overcoming that assumption of validity can be very difficult to do.
In most cases, it requires proving that someone pressured, unduly influenced in legal jargon, the testator into making a will in his or her favor.
However, since the deceased cannot be called into court to testify about any pressure, proving undue influence is difficult.
It is not an impossibility though, as the Wills, Trusts & Estates Prof Blog recently discussed in "Corpus Christi Court Upholds Undue Influence Verdict."
The article discusses one particular case where a Texas jury found undue influence.
That verdict was upheld by the court. While the details of the case are not necessarily important, what is important is that undue influence does not necessarily require definitive proof.
In fact, circumstantial evidence can support a finding of it.
If you think a will is invalid because of undue influence, consult with an estate attorney. Your case will be difficult, but it might not be impossible to prove.
Reference: Wills, Trusts & Estates Prof Blog (August 11, 2017) "Corpus Christi Court Upholds Undue Influence Verdict."