I’m not an expert on birds by any means, but I would think that it’s a combination of two things
1) Air gets thinner the higher you go. Birds rely on ‘lift’ in the same way as a plane (the shape of a plane’s wing is actually based on that of a bird). So, as the air gets thinner, they don’t get as much lift from it, so can only fly as high as there is enough lift to get them there.
2) There is less oxygen available as you go higher (related to the first point – the air is thinner, so there is less oxygen in a given volume). Birds rely on oxygen to breathe the same as us, so if they wen’t too high, they’d be unable to breathe.
There are probably other factors too, but hopefully that covers the basic points!
Hi joanna22! I think Stu covered the most important points. It’s worth adding that there’s probably not a lot for birds to do much higher up in the sky (except perhaps get a great view!!). All of their food, drink, and their nests are reasonably close to the ground 🙂