Because his playoff record is starting to become an outlier for 'franchise qbs.'
Peyton Manning was 3-6 before he won his first Super Bowl. People forget that along with his college career, Manning was viewed as a Playoff choker too. But that 3-6 record also included at least one conference championship loss.
Danny White, the original over achieving franchise QB before Romo and Dak, was 5-5 in Playoffs.
It's not just armchair quarterback fans and shock jocks calling him out on it, even the ex-players call it out in the media now. (although debatable if they're not trying to be shock jocks too)
Best comparison I can think of for Dak is Warren Moon at 3-7 lifetime in the Playoffs, and he didn't even get a chance to start in the NFL until almost age 30.