And that's the clear advantage of an active, and actively supported, modding community. Developers have a free pool of ideas to tap into and use as they see fit. It really surprises me that most developers fail to recognize the value of it. To me personally, it is a no-brainer.
It was clear to me that even as DLC were released for New Vegas, that small ideas developed within the community were finding their way into officially release products.
But different corporate philosophies, and visions and all...some are better than others.
Well, many developers use third party tools. As in... the vast majority. The cost of mod tools (the strongest and clearest way of supporting modding) is the inability to use any third party software. Which means your company has to compete against the best in the marketplace for audio rendering, for animation sequencing, for bug testing/reporting, for COUNTLESS other functions... using third party software lets video game companies be the best at making video games, while using the best tools in the industry (and beyond - the best animation tools used by Pixar, as an example, can be used hust as effectively as those used by a video game).
Housing and maintaining your own engine and support software is a huge enterprise that not only costs tons in development dollars and time, but also runs the risk of your games having a same feel to them... one look at Fallout and TES and you can clearly see the influence of what the Creation Engine feels like in terms of presentation and effectiveness.
So it's not just a matter of developers saying "silly modders... who cares about them?" It's a pretty nuanced decision, not just a matter of ignoring the benefits.