The answer, of course, is "money". People will go see this. Or if they don't, it's because they did a bad job of following the formulas. The summer blockbuster formula has worked out pretty well. People like watching stuff blow up, even if they could have predicted what would blow up and what it would look like before they paid $13 for a ticket.
Battleship took in $300,000,000. It cost $200,000,000 to make. That's "why". People recognized the name, and hoped to combine their love of stuff blowing up with their fond memories of a game they used to play. They get a little charge out of the connections. It's worth $13 and two hours of their time.
I could see this doing equally well. I can't say if it's the best use of the studio's quarter-billion-dollar investment, though it should be a reasonable one. It's more likely than some unknown script, which even if people really like it stands a very small chance of making more than $300 million without the extra name recognition.
I probably won't be seeing it. Maybe I will; I saw the Lego movie, and it was pretty good (though I paid no more than my Netflix monthly subscription fee for it). I'd rather see them spend their money on something with a bit more merit, but that's just me.