Why can't you highlight text in certain apps?

For example, in an Apple app, like the Messages app, you can copy an entire message, but you can't highlight individual words.

For a non-Apple app example, the Facebook app doesn't let you copy or highlight a post at all. It does let you copy an entire comment responding to a post, but again, won't let you highlight individual words.

What gives?

I'm on an iPhone 6 running iOS 8.1.2.