The only thing I can imagine why they made that decision is basically that they were too lazy or had not enough time to implement the model swap and additional proper animations for the 3rd person perspective. Which once again (with all the other missing and undercooked features on top) just proves how much they rushed this game and didn't focus enough on certain aspects in the game.
It was definitely not laziness. These people obviously worked like crazy. It was simply a strategic decision. They didn't want anybody to play this thing as third person, because "immersion". And I guess everyone who made this decision was convinced it was the right one.
The only problem is that not all people think, feel, or experience computer games the same way the people who made the decision do. However, high level decision makers rarely have the ability to acknowledge this.