Sooner or later, you are going to run into a worksheet where you can't see the whole worksheet going across even though you would like to.
Notice in the white box in the first row that it shows 100%. You are looking at the screen without any reduction in size.
One way to reduce the view is to click on the down arrow and change the percentage of the view. 75% means it will be shrunk to 75% of the normal size, so you can see more on the screen.
However, you can customize your view more than that. Another way is to highlight the columns that you want to see and then click selection. Excel will determine the amount the view needs to be shrunk.
Yet another way is to type in the exact percentage you want in the top of the view percentage box.
See the whole picture. Use one of these techniques to get the view you want.