I mean in any case you need platform specific http client, for js you can use only particular API, there is no way to write http client from scratch. So such multiplatform library will use the same approach, common api that delegates work to platform specific apis and libraries
But I agree, some standart common http client would be really helpful.
#ktor already have some common parts for http client and server, you cannot use them now for multiplatform, but I think it will evolve to some separate multiplatform library