It doesn't necessarily result in better games, but from a technological point of view, there's often a significant impact. God of War is a good example, as the game's data streaming is so extremely optimized for the PS4 that there's virtually no loading time once you're in the game world. For a game of that visual quality, detail and scale that's an incredibly impressive achievement. They would simply not get that running to the same standard without that laser focus on a single platform. Ghost of Tsushima's insanely fast loading times are another example, especially when compared to the excruciating loading times of a multiplatform title like AC: Odyssey.
From a creative point of view, you're right of course. These guys might've made these games anyway if they hadn't been first party and they might not have had to compromise on their artistic vision. But there would be a clear disadvantage on the technical side of things. How important that is, is for every individual to decide, but I would argue these games have clearly benefited from being exclusive to one platform.
If everything is platform agnostic, you lose that extreme optimization. The trade-off of course being that anyone could run anything on whatever system they want. As someone who'd rather get multiple platforms, I prefer having those games that run flawlessly on a single platform, but that's just my personal and very selfish preference.