Why are the objective lens of macro lens so deeply recessed in lens barrel?

As the title states.
Using my Tamron 90mm macro lens as an example, the first lens element is buried inside abt 1/3 the length of the lens...I see that the majority of macro lens are like this, any reasons behind this???

I have three guesses, better shielding from light sources close to the lens, better positioning of ring lights, and to protect the front element as they're the type of lens more likely to get poked by stuff.

I think for better positioning of macro flash is a very valid reason...but the other 2 could be solved by a lens hood...
Some how I don't think the hollow barrel is just to act as a built in lens hood, as it is so thick, lens makers won't waste so much material to make a built in lens hood...

You can't have a lens hood on for protection if you're using a ring flash though. I'm sure there is some more meaningful reason behind it, would be interesting to hear from someone who's disassembled one to see if the extra space contains anything.

If there's anything in that extra space, we could probably see it
just looked other lens diagrams of macro lens...found that only the 90mm version has this design, the 60 and 180mm macros aren't like this....this is getting weird!
Maybe I should write Tamron an email and ask this (pretty sure i'm gonna be laughed at by the staff!!!)

My Tokina 35mm and Sigma 70mm both have recessed front elements and I do like it, for those short lenses getting up close can run the risk of hitting things so having a perminant "hood" helps. However my 65mm and 150mm macros don't have that at all, the lens is flush with the front of the lens or near enough to.
On the 150mm its not a problem as I can put the hood on as normal and still get to 1:1 without problems - with the 65mm its a special case that it focuses crazy close at 5:1 so chances are a recessed design would have been counter productive to such close working distances.

The relatively new Canon 100mm "L" macro lens seems to have its front lens element positioned quite close to the outer edges of the lens. Just take a look at the bhphotovideo.com photos or other readily available photos. As I undergo "lens lust" re upgrading to the "L" lens from my "non-L": Canon 100mm macro lens, I worry about the "L" lens front element being more vulnerable to accidental scratching, because of its position near the lens barrel front edges...

Perhaps there are secondary factors that influence how much lens barrel overhang there is near a lens front element?

Well you can also make the barrel longer, but you can only make it so short. For example if you were to make the Tamron short enough so that the front element isn't recessed you'd pretty much lose the focus ring.

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I have the 90mm Tamron, it's rather annoying really, you can't use the sun cap thing, nor can you use a regular popup or gun flash, you need proper ring flash, maybe it's intentional who knows...

Actually if you build a custom "snoot" type adaptor for the popup flash, to direct the light forward and then have a flap at the end to direct it down over the subject you can get good enough light for 1:1 work out of the popup flash.

Just throwing this out there... but is the front element that retracted at all focus settings?

Yes, it is, the hollow lens barrel moves in and out with the front element, that adds to the question, as the focus motor has to do extra work to move the whole unit.