Someone could also just use your phone.
Same as having someone else buy beer for you.
This isn't a panacea.
It's a peculiar feature of this system that there is no way for anyone to ever identify the person who provided the proof that they are over 18.
Presumably you are doing the age verification to provide some service to someone. If you can't ever identify the person who provided the proof that they are over 18, how can you ever confirm that you are providing that service to them and not to someone else?
With regular IDs like a passport, you can always (if you kept a record of the ID) track down the person to whom the ID belongs and investigate whether the service was really provided to them.
Presumably you are doing the age verification to provide some service to someone. If you can't ever identify the person who provided the proof that they are over 18, how can you ever confirm that you are providing that service to them and not to someone else?
With regular IDs like a passport, you can always (if you kept a record of the ID) track down the person to whom the ID belongs and investigate whether the service was really provided to them.