Google never markets ATV at all, I have legit never saw an ad for Android TV. Also, it seems like there is no native Android TV apps, and if there is, they haven't been updated in forever. Any idea what could be the reason/cause for ATV seeming so "dead"/under-developed?
Because Google is doing a poor job of promoting their own platforms. Tablets, watches and TV fail to get any traction with devs or users. It's a chicken and egg kind of problem but I think Google's lack of dev support is the biggest factor. All the normal UI APIs are built around smartphones. So apps on tablets look like giant smartphone apps with tiny buttons and text, smartwatch apps get ignored altogether and TV requires UI that is scaled like a phone but on displays similar to tablets BUT without touch.
Google's laissez-faire attitude toward their platforms is killing their ability to compete anywhere but with phones.