Why couldn’t kings adopt an heir from a relative?

Beginning to learn about European history (with a bit of Russia mixed in. Not sure if Russia is Europe or Asia…)

A common theme is king so-and-so only has daughters, so he has to dispose of his wife, get a new wife and try again. But… if the entire point of marrying a Royal from another country was to secure relations with that country, wouldn’t disposing of your wife cause international tensions?

And why did the king have to be the father of his heir? Often, these kings have brothers who have sons and/or illegitimate sons of their own. Why can’t they adopt one of them and raise them as their own and make them heir? They would be related, so the dynasty would be intact. The king is raising/parenting them as he sees fit so no difference there. And the biological parents can hardly complain because they can have more kids and their heads remained attached to their necks.