Why does so much software use gray backgrounds, from Windows to Chrome to Mac to almost every other software and website you come across?

Because it's a neutral color that doesn't strain the eye and allows for good legibility when using black or white fonts, and its neutrality aids to place the focus on the real content window. The shade of gray will vary depending on the software, of course, ranging from a light gray to a very dark gray.

1. Why is the background gray?
2. How do I change a GREY background to white in Photoshop?

Why is the background gray?

It could be due to corrupt system files or missing/corrupt drivers or missing updates. I suggest you to try the steps provided below and check if it helps. Restart the computer a few times and check for Windows Updates. In the Search box on the taskbar, enter Windows Update, select Windows Update.

How do I change a GREY background to white in Photoshop?

In the Layers Panel, click on the Create New Fill or Adjustment Layer. Select Solid Color from the top and choose the color you want your new background to be. Now how do I change the background color to white in Photoshop? Simply pick the color picker, choose white, and you're good to go!