"We?"
If you do not wish to waste your time on checking the validity of ideas from newbs then that is totally ok and it is your choice. If someone else is willing to humour a rookie then kudos to them. I'm not saying that scientists must spend all their days explaining why the theory that moon is made of cheese is wrong (In fact ideas like that can be checked by the rookie himself/herself from information freely available to everyone.) but there really have been scientific discoveries made by rookies (or even teenagers) and disregarding everyone who isn't an established scientist will also harm the advancement of knowledge.
*sigh* I'm breaking one of my own rules here by attributing to a community what is merely my own opinion...
Yes, reluctantly, "We".
A prerequisite for solving a problem is knowing how to formulate it. In the case of cosmology it can't be done without understanding
General Relativity and its predictions for the interactions of gravitation with radiation and matter. Another prerequisite for solving a problem—albeit not a formal one—is knowing what is plausible and what has already been tried. Quite a few of the attacks to problems in cosmology invoke subatomic particles in some fashion (this is an understatement), and that means one has to know at least a little
Quantum Field Theory, a notoriously forbidding and treacherous subject. These two subjects aren't the type that you can just pick up in a year and a day; they are usually taught in physics graduate school over several terms. But if you want to be a theoretical physicist, having an acquaintance with them is nonnegotiable.
Now, as to the "we". Physics researchers literally get scores of emails from people asking them to examine their ideas. There just isn't the time to deconstruct every single one. So, like everyone else with bounded rationality, they use heuristic tests to decide which ones are important—having credentials, certainly, but also demonstrating command of existing techniques. And the emails almost invariably fail the latter.
I should probably disclose that I am a graduate student studying theoretical condensed matter physics (and not, in particular, cosmology, but I think my statements survive scrutiny).
[edited for clarity]