Money doesn't cleanly convert into time.
Having juniors and mid-levels is about being able to promote an existing mid-level that knows the team and the system, with zero downtime. It's much easier to replace a junior than a senior because of the lower expectations and risk.
Furthermore, a lot of companies are struggling to hire right now because the market conditions creates a flood of applications and its quite hard to discern who's a waste of time or not which leads to hiring processes taking longer.