As an engineering major I can tell you that:
-A regular plastic spoon would likely snap in the ice cream
-The square shape is easy to produce/ship/hold in your hand
-The hollowness saves plastic and insulates against cold because it holds static air
-the added length makes the above more effective/comfortable

as someone who worked at mcdonald’s i can tell you that:

we fill the cup with ice cream, we dump the oreos on top, we stick the spoon in, we put the entire thing in a machine and use the spoon to stir your mcflurry

bam

now that i’ve told you, they are no doubt coming for me. this is a top secret mcdonalds secret and i will surely be assasinated moments after i post this. spread this, remember this. tell them.

tell them all

godspeed

never in my entire life has a mcdonalds employee stirred my mcflurry for me