Why do people still buy DVDs even though they aren't even in HD?

DVDs are in 720 x 480, they aren't hd. HD DVD failed and Blu-Ray is king yet DVDs still haven't been discontinued. Are people really so cheap that they'll buy standard definition movies just because they're cheaper. The bitrate is only 6-8mbps and the new 4k format gets bitrate in the 50s and 60s.
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