If my PCS date is 14 Jan 2018, can I PCS 28 days before that without adjusting DEROS?

You must adjust your deros first, i understand people say just take leave however your DEROS date is the first day you can start leave. You cannot depart your current assignment any earlier then your deros not even on pcs leave. The good news is this am easy fix, after you have a leave form that shows you are taking 30 days pcs leave or if you just want to leave 28 days early the assignments section can change your deros for anything less then 30 days. So to sum this up the easiest route to go is speak with your assignment section and have them adjust your deros based off your leave form. Next option is do a 4187 which only has to be signed by the brigade commander to adjust deros 28 days.

Why don't you just take the PCS leave prior to the date?

There's no such thing as a PCS date, so I'm not exactly sure what you're asking specifically. But, the way it works is that you get your assignment, choose what day you will report by, back up the amount of days of leave you want to take, and submit your leave. Once that's done, your S1 changes your DEROS to match and travel cuts your ticket based off that.