Why here is using the present perfect?
Whenever the Powerball jackpot gets big enough, social media becomes inundated with fakes. Sometimes it’s just liars claiming to have won the prize. But other times, it’s liars claiming to have won the prize who want to share some of it with you. As you can see from the Twitter screenshot above we have a case of the latter with one of this year’s Powerball wins.

Multiple accounts HAVE SHARED the image, with some claiming that with enough retweets or shares, they’ll give some of the winnings to random strangers. The only problem is that the ticket is a photoshopped fake.

Question

Why here is using the present perfect?
Whenever the Powerball jackpot gets big enough, social media becomes inundated with fakes. Sometimes it’s just liars claiming to have won the prize. But other times, it’s liars claiming to have won the prize who want to share some of it with you. As you can see from the Twitter screenshot above we have a case of the latter with one of this year’s Powerball wins.

Multiple accounts HAVE SHARED   the image, with some claiming that with enough retweets or shares, they’ll give some of the winnings to random strangers. The only problem is that the ticket is a photoshopped fake.

Answer

The present perfect is used here to demonstrate that this, the sharing of the fake image, has happened in the past (i.e before ‘now’) , but without specifying exactly when.

Answer

'Because that's what the writer wanted to use' is the obvious answer. 
Grammatically-wise, I guess that it is a case of 'past continuing into the present'. The writer does not think that the sharing has stopped.