your chest insertions are just fine,your chest is flat because lacks of muscles so train hard and don't overcomplicated things.
and ,yes your question is not silly because most of us (me included) asked same question at the beginning,for years I thought to have genetic prepend (preposte) shoulders (slightly different from delts dominants) as my delts were bigger than my chest,no upper chest area,,,,I was taught from an heavyweight competitor at national level at my gym to do more flies than pressing "to stimulate more the pecs".
results; flat chest,overpowered by delts.
then I get bored and started to do just flat bb bench (supposed to hit just middle/low pecs).
after a while I discovered to have upper chest inserction...so I suppose that when your brain gives elettric impulse to a muscle it contracts whole not just a part.
at the end of the day,choose ANY (incline,decline,flat) pressing bench excercises,do it in good form,choose ANY reps range&rest periods and continue to add DISCS (excuse me, plates LOL) at the bar,when you hit a plateau simply change excercise (for example from decline to incline) and so on.
and eat to support muscle damnage you causes with your efforts.
PS; even some TOP bber (Cutler) always have lacked in upper chest,after years of efforts they fixed (almost) the problem,but it needs time (roids or not).....