On most platforms that aren't powerful games consoles or personal computers, the video decoding is done by proprietary hardware; these hardware decoders usually

On most platforms that aren't powerful games consoles or personal computers, the video decoding is done by proprietary hardware; these hardware decoders usually only support a few families of codecs and are difficult or impossible to reprogram. Even if most things support, say, H.264 out of the box, each video decoding SoC has different quirks which means either A) the same encoding looks different on different hardware or B) the encoding fails to decode on different hardware, so even the same codec can have a bunch of parameters which can/need to be adjusted for different devices. One smart TV might have a Broadcom SoC with a GPU which supports h.264 decoding like the Raspberry Pis do, an Android tablet might have a Mali GPU which can decode VP9 and HEVC, some weird older set top box which has netflix built in might only support MPEG...