Why? Licensing costs. Paramount Pictures and Warner Bros. protect their crown jewels aggressively. While you can find the film on services like Amazon Prime Video or Paramount+, those require subscriptions. Furthermore, many "free" tiers (like the ad-supported version of Pluto TV or Tubi) only offer the film in 720p or 1080p at best.

You cannot keep it forever, but you can watch it once for free.

Searching for " Interstellar legally is generally not possible, as the film is a premium title protected by copyright

You're looking for a way to watch "Interstellar" in full 4K.

While not "free" ($15-$25 a ticket), and not "at home," this is the only way to see the full 1.43:1 aspect ratio that Nolan shot. Streaming 4K cuts off the top and bottom of the frame. The theater does not.

: Interstellar famously utilizes "hard science," specifically gravitational time dilation near the black hole Gargantua, where one hour on Miller's planet equals seven years on Earth. Love as a Measurable Force