Because maybe you're personally just lucky and not having any symptoms for whatever reason? And because "just try it" can do real harm?
My anecdote for you: both of those are in the prenatal vitamins I take and have been taking for a while.... along with singulair, dulera, and my rescue inhaler. No positive change here.
Because my PFT showed the lungs of an 84 year old (twice my age) prior to treatment. Then after giving up treatment for self treatment for a while I had it done again and have lung function above average. Also, my decision was not on a whim, it was based on research. And finally, your prenatal vitamins do not contain much iodine - it should be milligrams, not micrograms (yep, I know about how much you're getting).
In this case, just try it should not do any harm. 250mg of magnesium is not a big deal. My doctor said iodine can screw up your thyroid - that's an old medical myth, but I've had mine checked a few times and the levels are very much in range.
So I stand by "just try it" along with YMMV because not everyones condition is the same.
That "old medical myth" has studies as recent as 2011/2012 showing that consumption of over 400 micrograms/daily of Iodine can cause subclinical hypothroidism [1]. That study needs to be confirmed, but given current recommended consumption in the 150-300 micrograms/daily range, recommending the consumption of 4x recommended levels or more as "should not do any harm" is strictly bad advice.
I'm stoked for you that you seem to have managed to treat your asthma problems with two mineral supplements, but please try to cite your research before trying to give medical advice.
My anecdote for you: both of those are in the prenatal vitamins I take and have been taking for a while.... along with singulair, dulera, and my rescue inhaler. No positive change here.