I imagine it's mainly that they're "sticking it to the man" and offering a product that is not only free for garage coders (Obsidian, et al. are required to purchase the pro version because they make more than $100k a year at being devs), but robust enough to make the enormous commercial engines start shaking in their boots. Or at least get off their asses and provide similar features, such as omni-platform support and web browser delivery (aka 80% of the bullet points Epic has been touting for Unreal the past few years).