Considering the advantages in viewing movies/videos with wide screen, why does Apple continue to stay with the old 4:3 for iPad?
16:9 is really only beneficial for video. Films are almost universally 2.37:1 these days (“21:9”) so they will not fill the screen of a 16:9 tablet anyway, and I do far more reading on my iPad (web, forums, rss, books) than I watch YouTube videos.
The 16:9 aspect ratio really forces tablets into landscape-only use. They’re horrible to use in the portrait orientation, which is primarily how the iPad was designed to be used. 4:3 works well in either orientation.
But what about photos, web browsing, PDFs?
Digital cameras almost universally have 3:2 aspect ratio sensors. This is closer to 4:3, and is the aspect ratio of the iPhone display. A lot of people seem to be cropping their images to be square these days, which is also much closer to 4:3.
Almost no websites are designed with 16:9 in mind, most are still a thin column of text, so any extra width is unnecessary. Even if you have the iPad in landscape mode, Safari’s reading mode won't fill the width of the display.
Paper documents (PDFs) and books, magazines etc. are all much closer to the 4:3 aspect ratio.
Most laptops are 16:10, and most people don’t fill the entire screen with the browser window.
16:9 is a legitimate screen ratio. The Grand Alliance spent millions in the 1980's studying what screen size made the most sense globally in terms of what would be the best new screen ratio (assuming we throw all old ratios out) for HDTV, and this was it. There was little compromise, and little dissension. The long and firm legacy of the Golden Rectangle ratio was just one of the issues contributing to the 16:9 decision.
Actually 16:9 is about as wide as you could physically make a CRT (even then, most 16:9 CRTs had problems with focus, convergence & distortion in the corners) and happened to be a nice intermediate between the current home displays (4:3) and cinema (21:9) which has people buying new TVs, but not cannibalising cinema tickets. (Well they got that part wrong)
4:3 +33% ≈ 16:9
16:9 +33% ≈ 21:9
16:10 works about as well here, but I tend to prefer the bit of extra room 16:9 gives you.
When we are talking displays, 16:9 is shorter than 16:10, not wider. The most common example would be 1920×1080 for 16:9 displays vs 1920×1200 for 16:10 displays.