I'm not sure where you're looking at, but there is two ways to get to Amazon Prime Video page;
In the header (underneath the logo), under the "Shop by Department" dropdown menu;
1) Shop by Department > Amazon Video
Or under "Your Account" dropdown menu;
2) Your Account > Your Prime Video
I don't find it difficult to find it following either one of the methods to navigate to it!
They have placed it right under "Your Account" to declare to the account holder that they've an Amazon Prime not to mention that it would be quite obvious for the user to guess!
The other link under "Shop by Department" is because they're still selling DVDs and movie related goods, so perhaps they can convince the user instead of spending so much on each DVD or whatever, they make the user to think "Ah it actually works out cheaper if I opt in for an Amazon Prime"...
And of course there are surely other marketing reasons behind it...