It might be doable, but it has a number of downsides. For one thing, it’d require storing 200MB+ per binary. Multiply that by the number of platforms you wish to support, and most packages are already over 1GB of storage requirements. Multiply that by the number of versions you want to support (at least one stable, and maybe one dev/master). Things get large, fast. I’m not sure if free Github plans support multiple GB stored for each repo under a single user/organization.
And even if each package did this, now you’d have to open a new Julia session for each package that you want to load quicker. Want to load 2 or more packages quickly? You have to hope someone else wanted exactly that combination of packages built into a binary together, and built for your platform.