It really depends on what kind of software you're building. A large new CRUD feature (which is what most of us are building 95% of the time) can easily be split among many people. But a complex data visualisation, or a novel algorithm, can easily take a month or two and isn't easily split amongst the team, even if you have multiple members with the expertise to implement it.

EDIT: Also, just because someone is working on a siloed task doesn't mean that nobody else can pick up where they left off. For that to be true you have to assume they document nothing and code like an idiot (which is unfortunately probably pretty common).