Everything up to the start-up is modeled, but the ignition itself is semi-automatic because these planes used all kinds of different starters to get the motor going: we'd end up with controls that would either have to be mapped specifically for just a couple of flyable aircraft, or having them only be usable via the mouse (which is not a big deal for something you do once at the start of each mission in my humble opinion, but many people dislike using click-pits).
Just as an example, to get the 109 running you'd have to do everything you already do plus on top of that:
1) Open your canopy or side window so your ground crew can listen to you
2) Tell your ground crew to spin up the starter.
3) Wait while they spin it up (the guy in black overalls standing on the wing and turning a hand-crank that's attached to the cowling like mad that we see in so many wartime videos? he's just spooling up the inertial starter)
4) Once they signal you or you judge by sound that the starter is going fast enough, engage the starter to the motor by pressing the relevant clutch controls.
5) Rinse and repeat if the start fails due to low temperature.
I'm all fine with it, but the complaining would reach epic proportions if something like that was modeled for CoD
So, we just press I and the sim simulates the sequence automatically,especially in the new beta patch startups take longer to complete now that we can hear the starter spooling up with the new sounds.
In other aircraft it's even more complex, for example the priming pumps and starter magneto switches for the Blenheim are actually on the outside of the engine cowlings