system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't I build a custom cellphone when I can build a superior laptop or computer for cheaper than most name-brand equipment?
gpt
The "you can" comments are missing a major component set that makes phones VERY different from your standard computer. RF and antenna matching gets really fuckey if you don't know how to tune it properly, which requires patience and relatively costly equipment.  If you are an RF engineer who has access to this (and the software/firmware for the major digital components,  cool).

Barring the analog and RF front ends- it's basically a computer.   Do-able, but tiny,  and therefore tough to pull everything together and make it play nice without many a ducat.  Very important - it also has to pass rigorous network and FCC specifications and go through what is called "type approval" with the major carrier/s.  

Now in terms of getting it all to play nice,  there is work being done to keep a standard interfacing between the different parts (see MIPI), but you will also be fighting various levels of firmware on the baseband/transceiver/apps processor,  depending on how the phone is configured.  

Ultimately, IMO, best-case scenario - you may have a decent pocket computer that sucks at making calls and accessing the network.