[rrwo]

A company like Twitter isn't made up of just "software engineers". Besides writing software, (likely different) people are deploying and running it, testing and integrating it with other systems, monitoring it, securing it.

Also, some of the people who have been working at a company for a long time know the systems in detail. They may not be the people who'd win a popularity contest, but you don't want these people to suddenly leave.