"Just because of their growth," Cuban said at The New York Times DealBook Conference. "I've had them for a long time. I don't switch things around. I don't move it around a lot."
Amazon, which has been disrupting several industries recently, is up more than 48 percent this year, according to FactSet. Netflix has risen more than 50 percent this year, and last month said it added 5.3 million net subscribers in its past quarter.
Cuban has previously proclaimed his fondness for Amazon before. In an interview with CNBC earlier this year, he said he thinks the online retail giant "is the greatest company in the world."
When asked about other investments he's made, Cuban said, "cash, Treasurys, municipalities ... stuff that I've had forever" and private companies "because I really try to support entrepreneurs."
"I'm the luckiest guy in the world," Cuban added. "All I got to do is not f--- it up."
Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank," which features Mark Cuban as a panelist.