When you're arguing against mainstream science, you're not arguing against one person with a PhD, you are arguing against the consensus of hundreds of people who have had decades of experience in the field...
That, unfortunately, is not always the case. Although the example is thought to be unique to economics (it isn't, but still...) we've recently had this whole dustup over austerity policies and the theory that a specific debt-to-income ratio results in stagnant growth. That basic assumption went unchallenged for years and was so widely accepted that whole countries began to make sweeping policy decisions based on it; imagine their collective embarrassment when that theory was exposed as bullshit by a graduate student.
Here's the thing: the mathematical proofs in cosmological physics have come so convoluted that no "conventional" explanation for those theories is even attempted; the model itself is exclusively expressed as a number salad that makes no sense to anyone outside of academia. This has troubling implications for intellectual honesty: researchers can and do fudge their calculations to get results they prefer, even in fields where laypeople CAN check their equations and potentially expose them as bullshit. If a highly respected physicist hacks together a bullshit mathematical equation that no one understands, who exactly is going to call him on it? Not his peers, who don't understand the equation and can't explain why it's bullshit. Not a layperson, who isn't expected to understand it and whose opinion can be dismissed. Not a graduate student, who can be shouted down as a crackpot or marginalized in grant searches.
Theoretical physics and cosmology both have this inherent problem: they operate at a very high level of sophistication in which it is fantastically difficult to independently verify their data, far more so than literally every other scientific field. What would really change about, for example, fundamental particle physics if it turned out that 90% of the experimental results over the last ten years were entirely made up?