The work these companies had was quite irregular, unstructured and unexpected as well. This demands a workforce that is very flexible, work over nights, can learn fast, can switch to any work easily (they are not stuck to a single kind of work role). Juniors wore many hats easily as situation demanded (faking included). They are also very mobile, can travel to a client site in any country. Most of them are singles. Feed them, do team outings, reward them and give gadgets (my team got first version of iPods free). They would give 50 hour weeks plus weekends. They hardly have any life outside of office. Sometimes they slept at office.
Also, the young workforce has a high team-bonding nature (romances included, intentionally). This makes them very good team players. What a manager means by "team player" is, they work for team goals, with poorly defined roles and tasks, without complaining.
There were a few seniors who review and guide the work, but the bulk of work was done by very mediocre, hard-working junior staff. When things go wrong, managers are ready to do crisis management and shuffle their young worker crowd across teams.
In other industries they are called MSGs: multi-skilled-gangs. These are very fluid work force. They don't know what kind of work will assigned to them on a certain day. It can be anything, from taking a support call, arranging lunch or pub party for the team, writing some html, testing some UI, giving a presentation to clients.
Undefined work requires undefined roles and employees that aren't yet married to certain work role, life style, or other person.