That’s because the problems facing Sony are multifaceted. Because of their low sales volume, each Xperia costs more to manufacture because Sony has to place smaller component orders to build their phones.
Sony Mobile Q3 2018 sales are beyond disastrous
Sohrab Osati

I would like to underline yet another component of their calamities you appear to omit, namely, Japanese endemic 6-month smartphone flagships refresh cycle instead of 9- to 12-month cycles elsewhere. Thus, Sony hi-end phones look fresh for a meagre couple of months after premiere at best, before next big tech show comes, and in 2018 retailers started to discount them especially fast and sharply.