Why are they waiting to bury the compromised reactor-sites?
There are several reasons for that:
1) They still need to keep cooling the reactors as the fuel is still active there. The fuel in three stricken reactors could still burn for a week or more. Without cooling the reactor's core could be breached and there would be massive pollution;
2) They need to ensure that nothing is leaking out of reactor, its cooling systems and storage of spent fuel. Origins of all leaks must be established and, if possible, sealed off;
3) In order to build a sarcophagus, one needs a project and technical design that could withstand earth quakes and tsunamis (they're still there). It takes time to craft one: the disaster site must be thoroughly studied, assessments made, engineering solutions worked out for many problems
4) Resources for construction: it will be a massive work associated with radiation hazard. It takes time to assemble what one needs for accomplishing it.
All in all, Fukushima will remain a problem for
years.