system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If a will is written by a competent, mentally stable person, and is handled by a lawyer and all things are in order, how can they be contested and overturned? Doesn't that defeat the purpose of the will?
gpt
#1  A will is ambulatory.  It has no legal effect until the person is dead.  Some states allow a testator (person whose will it is) to have the will be "approved" during his lifetime, thereby preventing any challenges to the will's execution after death.

#2  There are a lot of formalities that must be observed.  These formalities vary from state to state.  Some of them are silly, some of them are very serious.  You must follow the formalities because the state is divvying up your property after you are dead.  That is the price they exact for performing that service for you.

#3 If the formalities are not observed, there are a whole load of possible outcomes.  The whole will could be invalidated, or only part.  The invalidation could cause an older will that was revoked to be revived.  There are a lot of possible outcomes.  There are also certain provisions that are void, which once again may void the whole will.  Such as an "in terrorem" clause, which states that anyone challenging the will is disinherited.

#4  The purpose of a will is for a dead person to control the devise of his property.  America favours giving dead people great control.  You can disinherit your children if you want.  In other countries you cannot, and the courts will be obligated to give a certain chunk to the children - even if they abandoned their parents.  Anyways, since the purpose is to allow the deceased to exercise control AND the deceased is necessarily not there, the state wants to be damn sure that this is your will.

#5  Creating a new will, or doing a whole bunch of other acts, can serve to invalidate an older will.  The courts are very concerned about what is the true statement of the deceased.  

#6  Even if all of the formalities are observed, the courts can still invalidate a will if it can be shown that the will was procured by undue influence.  This is because a lot of old people in nursing homes who don't have any family have this odd habit of writing a will a couple of months before they die that leaves everything to the nursing home.

#7  Going back to #4, we see that the purpose is to devise the property as the deceased would have wanted.  Each state has a scheme by which property is devised if the person does not have a valid will at death.  These schemes are designed to mirror what the state believes the average person would want - this is called intestacy.  Therefore, if there is a problem with the will, and no valid will can be shown, then the state is at least comfortable that devising the property by intestacy should mirror what the average deceased would have wanted.

#8  We haven't even begun to discuss pragmatic concerns (dying means all of your assets are frozen, which isn't good if your wife doesn't have money for 8 months to buy groceries) nor the drafting concerns.  There can be huge issues with interpretation.  Or people devise things they no longer own.  So there can be all sorts of ways to challenge a will.

#9  Most states allow a "family settlement agreement" where all parties that have an interest (or those that may have an interest but aren't competent are represented in some way) agree to how *they* want to divvy up the property, thereby negating the will.  The purpose of most challenges is to hold up the distribution of the property so that a family settlement is come to.  Oftentimes it is much cheaper and quicker to do so.

#10 To limit the problem presented by #9, most states only allow a person who *could* take under intestacy or a prior will to challenge the current will.

#11 Wills are for suckers.  If you have over a million dollars in assets everything should be in trusts or corporations.  A true Estate planner will create more corporations and trusts than he will wills.