Maybe because it’s an early (older) codec which is/was most common. Speaking of compressed music usually first brought .mp3 (mpeg 1 layer 3) into mind. So there may be hardly a device available not being able of playing back such files.
Hm, yep - it tells which codec routine to start, so if the hardware FP2 audio decoder is capable of playing back .flac files it may do so if there is a driver with the OS to call up the routine. Otherwise a software decoder (provided by a player) is needed. You see, VLC can do it and many other players too. (I am using MX Player for all my .flac files).
Actually with Android 7 the hardware video decoder.H264 is not utilized by the OS due to the missing driver support from Google/Qualcomm. So there is only the cpu intense software decoder option by player apps available. A software solution should always work, but may turn out in a higher battery drain and probably lag at some point.