No, you are not stupid, you've just made a modelling error. In fact you overlooked part of the system.

When the rig is loaded the mast presses down on the mast foot which in turn pressed down on the beam AND the dolphinstriker pin. In turn the pin presses down on the dolphinstriker strap. The strap is ALOT more resistant to being pressed downward then the beam is. In fact the strap takes care of the vast majority of the mast foot loading; in the order of 75-90 % if the beam is not prebended upward initially. The strap comes under tension to resist the downward loading of the mast (compression in pin) and this results in almost equal compression forces in the mainbeam. The latter are transmitted from the strap to the mainbeam via the bolts securing the strap to the mainbeam near the hull.

This is commonly accepted stuff so far.

Now if the beam was perfectly flat before the mast being stepped then the beam will bend downwards when the rig is loaded up just like the D-striker and thus experience bending stresses. These bending stresses are highest on the top and bottom of the beam and typically an engineer want material in these places to limit these stresses.

That is your reasoning.

My reasoning and one that is commonly applied on cats is as follows.

If the d-striker strap is pretensioned upward before the mast is stepped then the mainbeam will straighten out first before being bend downward under a loaded up rig. Of course a straight beam can not experience bending stresses, only a curved beam can. So in effect if we give the beam the right amount of upward prebend we can lower the bending stress significantly under a fully loaded up rig. If the prebend upward is the same as the downward displacement of the V-tip on the D-striker under max sailing load then the bending stresses in the mainbeam are perfectly zero and the beam will ONLY be under compression loading. This latter situation is not always used in reality as it is advantagious to leave some bending of the beam but that is another topic. But I can give you some insight into the reasoning. The D-striker puts the beam under compression stresses and a slightly downward bending beam will see tension stresses on the underside of the beam. By careful selection of the upward mainbeam prebend you can have these two cancel one another out completely. In that situation it won't matter one bit how large the whole on the bottom as that part of the beam won't experience hardly any stresses at all ! The trick is to find the optimal average setting for a range of sailing conditions (rig loading)

And that answers the question

Wouter