There are plenty of mischievous things an attacker could do if you just verify each chunk separately, including reordering or omitting chunks, and truncating the file.
IIUC your question here boils down to "what is the point of a hash tree?" as opposed to e.g. a list of individual chunk hashes. The answer is that a hash tree lets you verify an individual chunk by looking at the hashes of O(log(N)) chunks rather than having to look at the hash of every chunk.