They've been trying for awhile (great point on ACG). I hated it when Adidas started making goggles and stole riders from Oakley. But, i realize it's just part of the capitalistic web. (Plus, they own Salomon.) If there's money to be made, then companies will gravitate towards it. And, if current companies aren't somehow filling the void effectively (technical innovation, stylistically, etc.), then it opens the door.
However, there are several barriers to entry in a tight-knit culture. In the end, it comes down to survival of the fittest, and being big or having lots of cash doesn't always make you the fittest.
Sure, Nike is doing quite well in skateboarding now, but their first two attempts failed. Nike SB's first run tanked, and Savier did miserably. However, Nike, Reebok and Adidas were the skate shoes of choice in the early 80s before Airwalk, Vision and others came along. Those roots paid huge dividends on Nike's latest run. The Dunk is the shoe that established the SB line, and the silhouette hasn't changed since the 70s or 80s. Now, retro/vintage stuff is in, and Nike's reaping the rewards of their heritage. Same goes for Adidas shell-toes. It was a little strategy and a little luck.
Meanwhile, Reebok's DGK and Ice Cream investments haven't helped them gain much ground. They'll keep trying until they decide the ROI is too low to continue. In the end, it's up to the consumer...