Because high-end chests count as ones opened with a key that's 600g or higher from a merchant (or all HM chests). You also can open it as many times as you have keys - talk about an easy (if not pricey) way to work on treasure hunter if it did count.
Zaishen chest gets you a different title, because it's very different from regular chests.
I don't know any "real" reasons why it does not count, but personally, I would not count the Zaishen chest as high-end because it is really easy to access. One could just map or even create a pvp character, withdraw lots of money, buy tons of keys, and just do a massive click combo on the chest. Which would also lead to the title,if the Zaishen chest were to count, to be easily spammable (which would bring up the whole debate as with the drunkard title change)
The main reason it doesn't count is simply because it already counts towards its own title, Zaishen Title Track.
There is only one Zaishen chest, and a limited number of ways to get the key to open it.
The high-end chests are random in location and they have the ability to be opened by a lockpick and if your luck is sufficiently high you can retain the lockpick to use again.
So, while you are opening a chest, the instances are different as PvE and PvP.
suggesting that it be corrected implies that there's a mistake
it is not a mistake that the z-chest is not considered a high-end chest, and there are a multitude of logical and obvious reasons why it is not considered one
This is getting a bit out of hand. :[ I would assume that the zchest counts for zaishen only and that's the reason why it's not for any other title. If you would still like to know, your best bet is to ask the dev team.