Theory: Because distribution companies for movies made having DTS/Dolby Atmos/Whatever mandatory in the theater, sound engineers primarily focus on that because that takes 99% of the time. Because theaters with just optical stereo (yes I'm aware most theaters use digital distribution and projectors now, that just hammers home the point) aren't really a thing any more they don't really bother optimizing it because "nobody will hear that anyway". Then when the film goes to home distribution the distribution company takes the automatic downmix that is the existing stereo track for the optical stereo on a print and use that because they don't want to second guess the sound engineers or the director.
Long theory short: it's the same reason the Beatles mono album versions sound a ton better than the stereo versions, because that was the optimized version because that's what people listened too at the time. Sound engineers aren't being paid to optimize the stereo or even 5.1 versions of the audio, only the theater version in whatever multi-channel standard is popular at the moment, so they don't bother they let the standard downmix stand.