Why can't you appropriate the territory of a state in an alliance?

[Vasek4444 1]

Hi,

You have a country and someone else's country they declare war on you or you declare war on them but they are either in an alliance or have a bond. You conquer all the territory ( even the territory of the vassal or alliance), you have 100% so you go to peace talks, but there you can only claim the territory of the first country, not that of the vassal or alliance. Why is that?

Thanks.