Voice actors don't all live near the studio they need to record at. So when they are needed to record their voices, they either do it offsite, or come in when their schedule permits it. Then you've got to deal with the company who hired the VAs. When do they need them to come in related to what the team is building right now? They may just want to focus on a single character at a time. IE, the first time Steve Downes and Jen Taylor met was during Halo 4's development, when 343i specifically wanted them in the same room.