Product review (1 stars): presumably to keep them from breaking so easily. I replaced all of the original knobs
Purchased this cooktop in Jan 2015.  Today (Mar 27, 2017) it is being replaced with a different product from a different manufacturer.  There are two issues.  First, the knobs are made of plastic and typically break within 30 days.  After installing the cooktop, we learned that GE had "redesigned" the knobs.  The "redesign" was to add a metal insert inside the plastic knobs, presumably to keep them from breaking so easily.  I replaced all of the original knobs, using the redesigned knobs.  Well cheap plastic is still cheap and the redesigned knobs broke almost as fast.  (Some lasted as long as 6 weeks.)  I contacted GE (Feb 2017) and they offered to provide a complete set of knobs at a deeply discounted price.  I agreed, hoping the replacements would be better.  They weren't.  Four to six weeks is all you can expect to get out of a set of knobs for this cooktop.  Secondly, and perhaps more dangerous, recently when cleaning the cooktop I removed the knobs and was washing the control area (around the knobs) with a washcloth when a few drops of water fell into the opening under the lock knob.  The electronics shorted out, sending flames two inches into the air.  Scary.  I've since checked other cooktops and discovered that the opening for control knobs is dramatically smaller, making it much harder for water to get inside.  Our previous cooktop was a GE that worked like a champ for 15 years.  Not sure what is happening with design and quality control at GE, but they are putting their reputation at risk with this cooktop.  Save yourself a bit of frustration and stay away from this one.