It's because I think respect of property in the moral sense requires some agreement, and there are times agreement has no meaning. It's consciousness that seems to give agreement any meaning. I do not know what an act of property transfer is aside from either physical grasp or some mutually agreed substitute. Basically I do not outright reject property being transferred via a will, but I reject it being identified as "property" much as the state cannot own the earth. A pile of compost might have once constituted a thinking being. Maybe a will is a tape recording or other artifact, but it's dead matter. It does not prove transfer takes place as somebody dies, because who is the pre-transfer owner? You have to refer to dead artifact, and then find out that person (as a rational agent) is simply not present. The compost pile's words may have residue of truth, but only the living can decide that. If they once wrote "2+2=4", we can carry out a testing process to distinguish the eternal truth from what's made-up. If they wrote "I transfer this", there's no way to check it by rational thought. Like the Bible, you can check that it was written, but you cannot check that what was written remains true today.
Agreement is not the basis of property. Property comes before agreements. In fact, property MUST come first, because agreement (i.e., a contract) is essentially a set of mutual promises to trade property rights -- If you do X with your property, then I'll do Y with my property.
Property that is based on mutual agreement is just another way of saying "social contract" -- you are essentially saying that property rights amount to: "You get to keep exclusive possession of your stuff only for as long as I agree not to take it from you."
That is the State in a nutshell. The bedrock beneath the Statists' claim is that "we" get to define what property is, what part of it you get to keep, how much of it you owe, and how you're going to hand it over.
Property does not arise by agreement, any more than the prohibition against murder arises by agreement. It's just wrong, no matter what you agree to.
A "property rights transfer" is consent. It's entirely mental -- a matter of pure intention. It's the difference between sex and rape. It's the difference between abandonment of property and theft. It's the difference between slavery and employment.
Property is transferred by pure intention, but that intention must be manifest in some observable way, so that others know what your intentions are. That can be any form of communication (even silence, in the proper context), or through non-verbal behavior. Property is transferred by pure mental decision, but that decision must (of course), by necessity, be deciphered by referring to some observable words or actions, or else no one can know what you intend.
Proving the intentions of a now-dead person can be difficult, of course. That's why the common law made all those (admittedly arbitrary) rules about what constitutes "proof." In wills, that's why the call the process of identifying and certifying a dead person's will "probate," which is just an old fashioned word for "prove." The objective is to prove (or disprove) that the document is a true, reliable expression of the deceased's intent, which is a lot easier to do if it conforms to some basic formalities, like being signed, dated, having 2 witnesses, etc.
But the deceased's intentions can be deciphered in other ways, even if there is no formal will. Any letters he might have written, statements made to third parties, even a video or a set of tweets. Yes, reviewing all of that stuff is something of a matter of interpretation, but the point of the process is to arrive at an evidence-based conclusion about what the deceased most likely decided, as to the disposition of his property upon death.
That conclusion might not be perfect, but it doesn't have to be perfect. It only needs to be better than all of the other conclusions about what the deceased wanted.
In order to respect property, you have to respect people's consent, intentions and wishes about their stuff. Property is a universal, objective ethical right, regardless of what other people might agree to. Otherwise, social life is just a free-for-all of Might Makes Right.