I'm sure the biggest issue is that it's a lot harder to fly under the existing zoning/regulatory umbrella if you build a structure. Most "micro-apartments" are significantly bigger than shipping containers (which are 320 sq. ft.) but even so, at least in Boston, their rents are still in the $2K/month range. It's really about the land price and how much vertical building there is. This amounts to unofficial squatting on industrial land. Of course it's going to be cheaper but it doesn't really solve any problem.
