Because the RTV was nearing the end of its shelf life
I sealed the galvanized metal wheel wells from underneath with silicone rubber (RTV) after the first long trip in pouring rain after it was obvious that the wheel wells needed sealing. That was almost 4 years ago, and there have, unfortunately, been many more trips in the pouring rain. Fortunately, the RTV has held as the leaks have never come back.
I took a look at everything a few months ago while installing the new axles, and the RTV does not appear to have debonded anywhere.
Note that galvanized metal, not aluminum, was involved.