They're not that readily available. At least, looking for OLED tablets there are very few. I love my Nexus 7, but its screen is so inferior for Netflix than my 4" S3 Mini. Even Samsung's flagship tablets don't use OLED. And OLED without naff Pentile matrices are even rarer. I think we've a long way to go to get decent mainstream OLED TVs. It was some years ago that Sony released the first small OLED. 2007 the 11" XEL-1, so sayeth Google. And with the push to 4k, surely more focus will be on higher res LCD than OLED.
Aren't the Galaxy S4 and 5 using 1080p OLED screens? They've been getting pretty good reviews.