I don’t know who thinks that but whoever does, thinks wrong. Despite having all the riches and resources within the geographical boundaries of Indian subcontinent, some Hindu rulers did try to conquer the neighboring lands and kingdoms and expand their rule outside India.
Many historians, however, believe that Indian rulers were never required to expand their rules to foreign lands or conquer them because it demanded much effort and bore no considerable fruits.
(Courtesy: The Official Home Page of the Indian Army)
To conquer neighboring lands like barren Tibet or mountainous Afghanistan would not have not given Indian kings any great benefit and would have demanded a big amount of army and finances. So Hindu rulers had always thought better of going and conquering such lands.
Even after all this, there were some kings who were ambitious enough to go out and conquer foreign lands. For example,
There was Kanishka of Kushana dynasty who extended his kingdom up to Central Asia. The Kushan Empire in his rule, consisted of Turkmenistan, Tajikistan, Kyrgyztan, Uzbekistan, and North West Asia.
Rajendra Chola of Chola Dynasty conquered Malaysia and Indonesia.
Although Shah Jahan was not Hindu, he also conquered Uzbekistan in his time. His rule in Central Asia, however, was a failure.