Mainly because of the chilling effect on speech this sort of thing causes. If you're on notice that your social media accounts will be examined at a border crossing, you'll be less likely to speak your mind regularly, if you have anything to say that's at all critical of the government in question. As these sorts of things become more normalized and pervasive, people feel less comfortable exercising their right to free speech, which makes it one of those things that are "only true in theory".
There's also the flaw in the idea that these things are related. If I say on a social media account something critical of the government or an elected official, that doesn't actually make me (me, personally, that is; I'm not speaking in generalities) any more likely to commit a crime or be involved in terrorism. However, a generic, statistics-based approach to flagging things like that assumes it does.
It's also not clear about what's public and what's private. My Facebook account is locked down so only my friends can see anything I post. Some people make their Twitter and Instagram accounts private. While that's not the same sort of "private" as "only exists on my computer's hard drive", it's still certainly not public in the same way that a comment on a HN thread is. If I give an immigration official my FB username, does that mean they get to look at my posts through a backdoor given to them by FB, even though all my content there is marked friends-only? If so, then I have a problem with that as it violates my due process rights (it should require a specific warrant, with my name on it, for FB to hand over that data). If not, then providing my FB account to them is useless, as they won't be able to get anything from it anyway[1].
I get your "limited resources" argument, but, to be blunt, I just don't care. I am firmly of the opinion that less effective law enforcement is a perfectly acceptable consequence of greater personal privacy. Fewer people in the US are killed by terrorism than by falling off a piece of furniture, so I'm not particularly concerned with trying to optimize the search for terrorists at the expense of due process and privacy protections.
[1] Actually that's not entirely true, as my friend list is public. So they could theoretically flag me based on someone I "know" (using scare quotes because a FB "friendship" tells you virtually nothing about the relationship between two people), which is ridiculous.