Ok i know it's a silly question but i'm really curious on this...
Is there a specific reason to wear the jersey in the pants or it's just more comfortable?
Maybe it's like McDonalds and is a required look for those that are paid to race. For me and the majority of people I ride with you'd be lucky to see matching gear :)
If I had to give a real answer I'd say it keeps it out of the way and stuff from going up your shirt when under way and minimize dirt rash when you crash.
The only guy I ever saw NOT wearing dirtbike gear while riding, was drunk and on a really old and small dirtbike. he even passed us once while talking on a cellphone and smoking a cigarette. kinda people that give us a bad name.
i wear the stuff because it's more comfortable, the pants are more suited to dirtbiking. tighter fitting so they don't get caught in stuff. the jersey breathes way better than a t-shirt would.