Because shipping containers are designed to move and these ones might have to.
TreeHugger emeritus Bonnie sends in a photograph of a pile of shipping containers on Lower Marsh behind London's Waterloo Station, which turns out to be a new "Apart-Hotel" for Stow, a London property developer. Stow-Away is "a new brand of design-led extended stay hotels" offering "a fun, distinctive and eco-friendly stay experience."
The site is, in fact, owned by the railway, and provides access to the viaduct behind. According to conservation documents, it was vacant because of "enemy action during the Second World War."
According to the architects, Doone Silver Kerr, "The proposed development uses 30ft shipping containers as the building system in order to meet the requirements of Network Rails asset protection agreement, which requires the scheme to be dismantled within 28 days."
Shipping containers are a bit narrow for decent hotel rooms; as you can see in the rendering here, there is not really enough room to get around the side of a queen-sized bed. Given the choice, most hotel designers would want a little more width. But if you have to vacate the site on 28 days notice, they make a lot of sense.
People who rave about the speed of shipping container construction should also remember that the biggest time suck in real estate is not necessarily the construction but also the approvals and financing and all the other stuff that goes into a building; this project was originally approved in 2012 with Will Alsop as architect. The new project is not identical; according to a local London site, "Doone Silver Architects say that plans for a roof terrace have been removed 'due to anti‐terrorism concerns'."
You can have the fastest construction system in the world and it can still take years to make a building.