Instead of starting over and building another browser rendering engine from scratch, they could have switched to Webkit, putting their development cycles to bet

Instead of starting over and building another browser rendering engine from scratch, they could have switched to Webkit, putting their development cycles to better use contributing to a mature, stable, Open Source browser. Thereby improving the web for everyone, and making their browser the better for it. Why is it commendable that they had such an opportunity and instead decided to re-invent the wheel (again)?