The real answer to OP's question:
"Because few people understand how summoners work, and fewer still play them to their fullest"

As a friend back in Guild Wars 2 once commented on Mesmers "I don't really understand what you're doing, but I notice when you don't" - when you have a summoner he waves his book about, lots of debuff icons and stuff melts. But you can't really put your finger down and say "here is the source of the DPS", unlike bard where you have a guy pew-pewing a stream of very visible arrows into a guys face, a dragoon ramming a lance up a bosses arse, or a black mage hurling giant fireballs.

A lack of transparency in a summoners role in melting mobs means they fly under people's DPS radar; further compounded by the fact summoner's don't parse very accurately