When your WR group is very strong, you don't necessarily need a prime time TE.
Knox can continue to develop into a solid starter, which is all the Bills need.
A good blocking No. 2 TE that can catch a few passes is also a need and the Bills think they have that now in Hollister.
I won't argue if you guys still want a dynamic player like Ertz, but the Bills WR position is so good, they really don't need Ertz.