It's not just as easy as that, there are various risk concerns involved in Bitcoin, it's different from other forms of physical assets were you can just draw up a will and after your demise, with the help of a lawyer, your beneficiaries can claim what you left them, in the case of Bitcoin, for that to be possible, you must give up your seed phrase, which is pretty difficult to do, as just with a misuse or a careless act, with that 12 or 24 seed phrase, your funds could be imported by another and lost for good.

Thus having said that, it's pretty difficult for people to disclose the 'key' to their wallets/crypto funds, except to one or two very trusted persons; ignorance is another reason why you don't get many people having beneficiaries to their funds, the one whom you can trust with your seed phrase may actually not be knowledgeable on the network, thus you run the risk of even losing your funds before you lose your life if they have access to your stash, that's why it's also pertinent to teach whoever you plan on making your crypto beneficiary one or two things about security on the network, so they don't lose the funds before or after you're even gone.