Why is that when one person or group gets a good or service, someone else will have to live without it? !

There's this statement in my Applied Economics book that I don't understand. Why is that when one person or group gets a good or service, someone else will have to live without it? I hope you answer this question. Thank you.
That statement is not generally true.

It is true of rivalrous goods (e.g. when I take a slice of a pizza, there's that much less left of the pizza for you).

It is false of non-rivalrous goods (e.g. when I step into the sunshine, there is usually* no less of the sunshine for you).

*Unless of course I somehow also block the sunshine for you.