The video game industry has a tendency to shift rapidly, as studios fold and new ones form, and some are acquired by larger publishers. Valve has remained independent for years, and according to founder Gabe Newell, it intends to keep it that way. He remarked that given the general trend of companies that are bought out, it's more likely that Valve would "disintegrate."
"It's way more likely we would head in that direction than say, 'Let’s find some giant company that wants to cash us out and wait two or three years to have our employment agreements terminate,'" Newell told the New York Times.
He may have a point. Companies that are bought sometimes lose a part of their identity, either by staff leaving voluntarily or the publisher cutting some staff members. Bizzare Creations was acquired by Activision, after which it produced a few more racing games before closing. Neversoft is best known for series like Tony Hawk and Guitar Hero, but has now been switched to Call of Duty.
Valve has a very free-form policy when it comes to its employee responsibilities, letting staff come and go to different projects where they'd feel most effective. If it lost that part of its identity, it may not be much like Valve anymore at all.