Mainly licensing. Unreal had always been big in the modding scene, and around 2009 they released Unreal Engine 3 as “UDK,” which was basically the release format we still have today that allowed any random person to use it.
Source, like other engines such as Frostbite and Decima, kept it’s licensing on a “contact us for licensing,” basis. You can download some modding tools, but if you want to use it for your game you need to jump through some hoops to get full access. This basically allows Valve to focus more on other things like games and Steam instead of treating their engine like a money maker itself.
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