Last tested: Apr 26, 2019
Because that is the timeout set in Looker's internal code. This includes all Action Hub scheduled jobs, which are done through webhooks. This timeout is not currently documented anywhere else.
It's possible that the action can still be successful, even if this error is being shown. This error represents Looker's connection to the Action Hub timing out.
The scheduled job "Schedule Name"[scheduled job number] failed for . Please visit your content ([content URL]) to view and resolve the error, or contact your Looker administrator if you need further assistance.
Error: Could not send webhook to "[action hub URL]". Net::ReadTimeout: Net::ReadTimeout. (Webhook ID: [webhook id])