Prepositions

[Falco]

why can`t we use ( in ) instead of ( on ) like in this sentece she is sitting (on) the stairs and not in...............or like there will be thunderstorms (in) the west of the country, and not (on) the west of the country...

[Tdol]

The stairs are a surface, so it is logical to be on them. When an area has a border or limit, like a garden or an area of a country (real or imaginary), then we use 'in' because we have the limit around us. When we think of stairs, we don't really think of the enclosed space between the wall and the bannisters, but what we tread on or sit on.