Scaling will do that. It has little effect on content like video (for games, for instance) but if you're capturing a lot of text, you're going to see edges soften if you do any scaling.
So I'm assuming your display's native resolution is 2560x1440. But OBS is set to have a canvas resolution of 1080p, so you're downscaling there.
Then, your output resolution is set to 720p. So you're downscaling again.
Try to limit the number of times and the amount you scale. The best would be to stream or record your native resolution; but 1440p streaming can be challenging, and if you set your display resolution down to match OBS' canvas or output resolution, that's ALSO scaling and will make the screen look fuzzy *to you*.
You might try keeping the native resolution but streaming at a low framerate.