Why so many Nepalis have their lastname "SHRESTHA" ?

Shrestha is nt a actual surname..it's jst an entitlement given to some newar community because they were very good in trade(that's what Shrestha means in Sanskrit ;best). Actually all the Shresthas have their own surnames..the panchthatiya Shresthas usually write Shrestha instead of actual surnames BT the chathariya Shrestha like amatya,joshi prefer using their own surnames..and those who use shrestha as surname if can't tell their middle surname( which is the actual surname) then believe it they aren't the actual Shrestha