Tabs switch to Accordion view automatically when there isn’t enough space to display all tab titles in one row.
To prevent this, go to Settings → Responsive and disable “Automatically switch to Accordion view” or select “Keep Regular Tabs.”
Keep in mind that in this case, tab titles may be cropped or wrap onto multiple lines.
You can also move the tabs widget to the bottom of the page (to give it more width), reduce the number of tabs, or shorten the tab titles.