Because those are the D&D rules, simple. If it aint broke, why fix it. Also because it makes sense..
Why any limits at all? Let everybody do everything. Barbarians can be mighty sneaky too! They should be able to Rage From Shadows and Backstab with a tree they just ripped out from the ground.
Limitations are Classism and thus Evul! and Fascist and BAD!
These are poor takes. The first one refers to an authority which immediately kills any fresh ideas and leaves no room for discussion.
The second is blowing the logical suggestion out of proportion and extrapolating it to some completely nonsensical and ridiculous examples.
Tell me, free of prejudice, can you make a sneaky headshot with a hand hammer from behind? A spear thrust in the back? How is that wrong?