Why is a paralyzed character incapacitated?

The paralyzed condition notes

A paralyzed creature is incapacitated (see the condition) and can’t move or speak.

Under incapacitated it says

An incapacitated creature can’t take actions or reactions.

I mistakenly thought that my character could issue mental commands (to control a spiritual weapon perhaps) while under the influence of a hold person spell. However, as has been noted on the site already, paralyzed creatures are incapacitated, and therefore can’t take actions, and therefore can’t take bonus actions.

My question is, what’s the justification for this rule? Why does it exist? What (in game) stops a paralyzed creature from taking an action, like casting a subtle spell, that requires no movement or speech? Or is the rule designed this way for balance or simplicity or something?