I would consider "junior" here not as a person in non-critical role who cannot function as an individual contributor, but as a solid candidate that lacks industry experience but can be at the moment hiring be expected to quite fast grow into the role of a solid IC. I.e to use a bit too stereotypical characterization - not a "stablehand" but a "journeyman apprentice".
At the moment of hiring you don't count on it that they can deliver (but guess and hope), whereas the seniors will have shipped products under their belt.
