Not all games are not mod friendly, it just tends to be larger companies and for good reasons.
As we progress, game engines are getting more and more powerful meaning they are also getting more complex. As the engines start to grow in complexity we start to see companies starting to use more and more 3rd party software, either as additions to what they have in an engine, or as tools they must use to do any work on the engine.
Battlefield gets a ton of flank for dropping mod support in the frostbite engine, but Dice simply can't release mod tools (or open the game up for modding), even if they wanted to. They are stopped from doing this by all of the 3rd party things they have to use. The licensing fees for that would by abysmal, and why should they have to cover that cost? I think an ideal situation would be them renting out modding licenses to individuals, but then they'd have to create the tools to do that, and I imagine that's just not worth the money.
Companies who own every aspect of their engine (or just don't have such complicated engines) are usually pretty good for releasing mod tools, or just enabling easy modding if that isn't feasable. These companies tend to make a lot more money because of the ability to mod and expand upon the game brings more people to it. I don't think that they worry about losing DLC sales much when it comes to modding. A tiny percentage of gamerpop plays with mods, and there are some games which allow modding but still release successful DLCs/expansions, Battlefield was one of those in the past.
