But communication is more than just the literal meaning of the words. I could say something in different ways because I want it to come across slightly differently, like:
Shoplifters should get a life sentence.
This is my opinion. I have spoken. Prove me wrong.
I think shoplifters should get a life sentence.
This is my opinion. I understand it's just what I think, and maybe I'm open to a sensible discussion.
Personally, I think shoplifters should get a life sentence.
Either: I've heard your opinion, and disagree. This is my opinion and I think it's better than yours.
Or: This is my opinion, but I want to emphasise that it's only a personal perspective, and that I don't imagine it's some objective truth that everyone should agree with.
In my opinion, shoplifters should get a life sentence.
I'm trying to sound ever so reasonable, informed and balanced.
In my personal opinion, shoplifters should get a life sentence.
Either: This is my opinion, but I want to cushion it upfront by using TWO words to doubly emphasise that I'm not trying to impose it on anyone else. I don't want too much backlash.
Or: I think I'll sound posher/smarter/more important/more formal if I add more words.
I'm not being funny but shoplifters should get a life sentence.
…I actually have no idea why people say this.
I get why OP is irritated, because once you notice a change like this, you start to see it everywhere, even though people used to get along fine not saying <whatever it is> every three minutes. But I disagree with your opinion that people should always stick to what's "sufficient". Redundancy can be annoying but it's not automatically functionless. There's lots of times people add extra words that say the same again, to add emphasis or adjust the meaning. Like the difference between
"My brother was caught for speeding last week" and
"My own brother was caught for speeding last week".
They mean exactly the same thing, but would be said in different contexts with different underlying meaning and emphasis.
And you say that you can't hold any opinion other than a personal one, but what about the distinction between personal opinion and e.g. professional opinion? When you've heard enough times about somebody getting a legal opinion on something, or talking about medical opinion or some kind of other opinion, then it might feel natural to use "my personal opinion" to signify what kind of opinion you're sharing.
It might also imply that this subject is somehow emotionally personal to you, maybe?