I for sure understand this. But smartphone sound is not something manufacturers care so much about, in my opinion. Smartphone is a multi-functional device and playing music is just one of many (not the most important
) functions they do.
So what i think is they do not waste a lot of time sweating over making sound very cool. It's a well-developed a-la best practices process... Building something from ready blocks (DAC, amp)
Not coping with cool implementation, as it happens in serious audio players. There is just no room for that in literal sense! Another thing: compare amount of produced smartphones and audio players. You think smartphone companies have enough time to play with sound tuning?
So if those blocks are the same - sound is more or less the same... But progress intrudes here, too - new audiochips in smartphones are more advanced than before, so they may produce a better sound than before (usually audio testing labs confirm that). And i do not argue here.