Wouldn't it be easier to just pump concrete over the reactor until it's all covered up with a solid concrete instead of hurting so many workers and putting so much effort and costs building a structure over it and cleaning up the area?
The likely reason is that simply pouring concrete over it would not have contained it.
The site is smoldering and emits gases of various types. It also has voids and shifts around, various areas collapsing into pockets. If it were covered with concrete, the concrete would develop cracks and radioactive particles would be emitted.
The sarcophagus structure is designed to be flexible and to seal everything in as tightly as possible without cracking, although it does have ventilation, so radioactive gases do escape. The structure also has viewing ports that allow the interior to be monitored.
A large amount of concrete was used in the construction of the sarcophagus.
The original sarcophagus built around 1986 has some issues and it was estimated in 1988 that it would only last 20-30 years (here we are). Check out this link that explains some of the main issues with the current sarcophagus; namely structural instability. Trying to repair the current sarcophagus is impractical.
The new confinement structure is designed to help shield the site from weather (e.g. rain water drains through the reactor picking up radiocative materials and then leaking into the groundwater). It also allows workers to partially dismantle and clean up what is there, which is a better long term solution. Here is a link about the new confinement structure.