Tablets mostly spend their energy budget (under normal operation) on illuminating the screen and most of the action content they see would either classify as webpages or video encoded with codecs they have hardware support for decoding efficiently.
A generic PC meanwhile runs a heavy OS and is often asked to do large volumes of local processing tasks. E.G. that bloody 16GB Outlook mail database that has to be stored locally because your power user that demanded a laptop has to be able to read all of their past email while they're in the field with no reliable net connection.
Oh, they also demand that it has to be light. Fans are cheaper and lighter than putting enough metal inside to do something passively; and also run cooler since even a pathetic volume of airflow is many times better than waiting for convection alone to move the heat.
You can blame the noise of the fan on the push for crazy flat laptops though. There's no reason a still light but thicker laptop couldn't use a larger slower moving fan. Well, other than the perception of thick as heavy and thus bad.