There are situations where a person can not give consent, despite being asked by someone to give consent. Consenting to be killed is often defaulted to being one of these situations.
On top of what @JohnDallman mentioned, there would be difficulties confirming that consent was, in fact, affirmatively given - and not, for example, given coercively under duress. If, for example, you had video evidence of consent, you might have the person who gave consent confirm that the person behind the camera didn't have a weapon and intimidated the person into the statement of consent, or a threat wasn't being made before the recorded timing of the consent being played, etc.
In a similar matter, there are situations where someone could try and give non-coerced consent, but be physically incapable of giving consent. Probably the most obvious situation where this comes up is the concept of an Age of Consent; this usually applies to consent to sexual activity, but the rule essentially says any consent by a person below that age is considered invalid, regardless of how much documentation you have of them asserting consent. But beyond that, for example, if someone is incapacitated with alcohol (Or with drinks that are unexpectedly spiked) - any consent would be considered invalid, even if the person is above the Age of Consent. These are barriers that prevent any consent being gathered from being actually valid.
As a result, generally speaking, consenting to be killed is usually considered by default to be in these given categories; you'd have to prove that you weren't coerced into the consent (Which you can't do when you're dead), and if that could be, you can't prove that the person who was killed was of sound mind and wasn't incapacitated at the time of giving consent.
The part about presuming the lack of valid consent is why, even in jurisdictions with euthanasia, there's concerns about the final degree of consent when asking for the procedure, leading to very specific rules about having to have final consent explicitly waivered (In a way that can still be waived at a later time by demonstration of refusal or resistance) while they're considered to still have "Decision-making capacity" well before it is granted, but are expected to, or fear, losing said capacity before the procedure is performed, and it has been assessed by specialists in the reason for requesting it, and approved before the waiver is asked for, if the date of the procedure is beyond that.