Because they have to do only one thing, and literally billions are produced every year, so if qualcomm spends 1 billion for R&D, the modem price will only need to be $1 "higher" to cover the r&d cost... if SDR development costs $1 mio (and that's basically zero for hardware design), and 10k units are sold, that's $100 per device in R&D cost.
(numbers simplified and rounded to make an example)