Summarizing the other answers and comments I think I can see how this is a solution to a structural problem. I'll outline it here so others can validate it:

The macro lens in question has two element groups: one fixed in the rear barrel, the other fixed in the front barrel. Focused at infinity the distance between front and rear glass is 2.2". At its nearest focus the distance between front and rear glass is about 4.2".

If the lens were built to be just over 2.2" long, then when the front barrel extended to 4.2" there would be practically no overlap with the rear barrel or the focus ring. I.e., the front barrel would be virtually falling off the end of the focus ring and rear barrel at that point.

By extending the tubes 1.5" past the front element the overlap at full extension goes from about zero to 1.5". If they optimized that length I would guess that's somewhere in the range of the plastic's bending/cracking point, so if the lens is bumped/dropped at full extension it's as prone to break as to pop the front tube out.

The 300mm telephoto shown for comparison goes from 5" long to 7" long, so there is still up to 3" of internal overlap/grip on the front barrel at full extension.

Note that this question refers to "block focusing" lenses, which are typically on the cheap end of the spectrum. Internal focus designs do not change overall length and so we would not expect to see deeply recessed front elements on such designs.