He likely could. I'd speculate a couple of reasons for not taking investment:
1) Control over IP and final product. While some investors would likely give free reign to continue as he has been, identifying and coming to an agreement with them might seem time consuming, which leads to:
2) Doing business takes time, he may feel that time is better spent finishing the game.
There's a good chance there are more nuanced details to his decision to borrow instead of seek investment like project progress and terms of his loan.
1) Control over IP and final product. While some investors would likely give free reign to continue as he has been, identifying and coming to an agreement with them might seem time consuming, which leads to:
2) Doing business takes time, he may feel that time is better spent finishing the game.
There's a good chance there are more nuanced details to his decision to borrow instead of seek investment like project progress and terms of his loan.