I think he just didn't place enough value on it. He would get some depth but it never seemed like enough. This contributed in some part to the team not meeting expectations in 2013 and 2014. In 2015, Toronto was extremely lucky in the health department IIRC. They got 28+ starts from 4 different SP, which is pretty crazy. So was Stroman's miracle recovery.
Did a SP even get hurt outside of Stroman? I guess Buehrle's shoulder wore out.
He also had a weird habit of bringing guys into the org and then never really giving them a chance. Or cutting them loose after 1 big league innings, etc.