I've mentioned this a couple times before ....
Thompson is the only quarterback on our roster with significant playing time. BTW, none of our quarterbacks have played on teams that have "done squat."
The issue as to why experience matters is because Frost is on the hot seat, with a contract structured to release him during or following the season. It may be that Smothers, Purdy, or Haarberg have higher ceilings than Thompson -- but there's also a bunch more unknown with all of them. The staff doesn't have the luxury of looking for a QB to build/develop for the future. Their jobs are on the line. They must win now. So unless Purdy, Smothers, or Haarberg looks clearly better than Thompson, they can't chance it. Granted, if Thompson is still injured in a couple months, they may be forced to go another direction.