The way DNS works in i2p makes it unreliable and vulnerable to attacks. It wouldn’t be to hard for an adversary to do a man in the middle or even do a fake version of a site. Also resolving DNS names is hard and takes a lot of effort.
Honestly the entire system needs to be rethought.
I think the reason it isn’t like that is because it is incredibly complex to do. Also if there is a design flaw it could be used to attack people.