This is fast time simulation, not real time simulation. When it comes to realism, there is a limit as to how realistic a simulation can be and retain its practicality.
In reality, the airlines have huge amounts of time to make decisions about which frames to buy, this is often based on a viability/feasibility study per route, business case for a/c type against a/c type and so on. We don't even closely replicate those processes. It is a thumb-suck at best with little risk of failure (other than greediness causing you to have too many a/c and no routes to fly them on).
This game we play is nice and to be honest is pretty close to a good (high level) replication of reality but it remains a game and, in order to make it half/decent to play, there must be variations from reality.
PS : Sorry if I drifted off-topic slightly