Traders don't stand and fight. Fighters do. There is a key difference in playstyles here, and a single NPC difficulty level could accommodate both playstyles. Traders don't need to be able to take a lot of damage.
But in fact you can spec them quite easily to take way more damage than a combat fitted ship and still haul a tremendous amount of cargo:
T6
653 MJ of health (2.5 times my Viper)
85 % of the speed
92 T Cargo Capacity
http://coriolis.io/outfit/type_6_tr...45020127.Iw1-kA==.Aw1-kA==?bn=Blockade Runner
T7
930 MJ of health (1.2 times my Vulture)
208 T Cargo Capacity
http://coriolis.io/outfit/type_7_tr...03030505040403452727.Iw18aQ==.Aw18aQ==?bn=T-7
T9
1232 MJ of Health (107% that of my FDL)
496 T Cargo Capacity
http://coriolis.io/outfit/type_9_he...a0303020227.Iw18eQ==.Aw18eQ==?bn=Beefy Trader
Comparing shield strengths with combat ships doesn't really make any sense. Fundamentally, a T6, T7 and T9 are significantly bigger targets and have significantly less maneuverability than fighters and thus take a lot more hits overall than any fighter will. And with the exception of the T9, those other trade ships also suffer from slow overall top speed compared to combat ships, meaning getting interdicted by combat NPCs can easily end with those trade ships being destroyed before they can high wake.