It depends which people we are talking about. For Native Alaskans this apparently was obvious.

Now if we are talking about Europeans, then the answer that Alaska was explored exactly because people in Europe wanted to know how big the continent of Americas is.

First Europeans who visited Alaska were members of Russian Great Northern Expedition

In 1724 Peter the Great issued order of organising an expedition which had the following goals:

prepare one-two ships in Kamchatka or nearby; using those ships, explore northern lands, which seemed to be part of America; seek where those lands join America, and whether there are any cities in European possession in the area. When meeting European ships, it should inquire them the names of the local geographical features, explore the coasts on the way and map them.

Expedition was led by Vitus Bering. First expedition ddin't reach Alaska, but the second time Bering reached Alaska.