Why does caffeine exist in chocolate bars? Why can't caffeine only exist in soda, coffee, and tea etc?

Caffeine is a naturally occurring substance in cocoa solids that are used to make chocolate.

Caffeine is not added to chocolate bars, it is naturally present in the cocoa/cacao bean which is used to make chocolate; both chocolate drinks and chocolate bars. However, the amount of caffeine in cocoa is lower than in coffee or tea.

PS If you like the taste of chocolate but don't want the caffeine, you could try carob from your local health food store. This has a similar taste to chocolate but is caffeine-free, and both powdered forms to make drinks and for flavouring, and bar forms to eat are available. The minus side is that it has about the same number of calories as chocolate.

Because it does.  Caffeine naturally occurs in over 2000 foods