It wouldn't be for the same reason that someone picking up the spare key from under your mat and letting themselves in wouldn't be 'Breaking and Entering' since there is no 'breaking' involved.
To qualify as hacking requires some level of technical work to defeat,bypass,override or otherwise circumvent security measures. Reading an unsecured file, then typing in the plain text username and password found within it, hardly qualifies as any of those I would say.
Not saying it's morally or legally right, anymore than someone letting themselves into your house with the poorly hidden spare key is.
But calling the perpetrator a hacker is like calling the person who used your key "a highly intelligent and elusive cat burgler".