Even if the source is kept decently preserved, the build environments are usually not. If they still have a machine in the exact state it was in at the time the game was finished, it might be as easy as Project -> Build, but... they almost certainly don't. So that likely has to be rebuilt from scratch, and you'd be very lucky to find any kind of documentation on how things worked.
Game studios tend to have it particularly bad because of how much binary-only engines/middleware (standalone bits like Havok physics/Bink video/etc) they used, how often the game's data and code builds were mixed together in some way and how in some cases the project is designed to build things like console releases at the same time. If you lost the install files for your physics engine, you're probably straight up screwed.
By the time you've figured all of that out, you can be easily hundreds of hours in, with tons of weird little issues that might require different people to solve. Some examples: you might end up needing to build it in Windows XP because no other OS runs all of the software used during the build, any sysadmin is NOT going to be happy installing WinXP on their network so the machine has to stay offline, getting code onto that machine might be a pain due to how Perforce or whatever is used by them, even things taken for granted like a particular version of the DirectX 9 SDK might be hard to find, etc. Sometimes licensing/activation of tools used in the build process is an impossible to solve problem because it needs some DRM dongle or activation server that no longer exists and the software was never publicly available, so there is no crack.