A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
It appears that the problem is caused by the manner in which you're selecting the additional cells - The video suggests that you're holding the Command key while clicking. That selects each cell individually which prevents them from being merged.
To select multiple cells as a single selection among other options either:
- Drag across them, or
- Select the first then Shift+Click the last, or
- Select the first then use Shift+→ to select the rest
BTW: Merging cells is something that most experienced users will advise against. It alters the integrity of the grid & can present any number of issues with the use of other features, as well as corrupting the workbook. If you do use it, keep it to a bare minimum in any workbook which will be used on a working basis for storing data. (It can be used rather safely when designing a form layout for printing. For example, when I was Secretary of my dart league I designed scoresheets which used the features but strictly for printing the sheets which were filled in by hand.)
For the effect you are intending try selecting the cells then going to Format> Cells - Alignment to select Center Across Selection from the Horizontal alignment dropdown.