Because not all amendments are created equal thus the reason why some come and go over time. The Second Amendment already has numerous conditions attached to it that technically would be against it but I'm pretty much okay with not being able to go to the 7-Eleven and buying a nuke.
Actually dean the Constitution ensures all amendments are "created equal". The reason why some "come and go" over time is that the citizens of this nation wish them to. ANY amendment has the same ability to "come and go" in the same manner any other does. Once again the Constitution ensures that.
You can say the word fire in a crowed movie theater if you are simply talking a bout a campfire or even the neighbors house fire. It is illegal to shout "fire" in a movie theater if the intent is to cause a panic which can injure people.
So how then is the correlation between limiting fire arms because they hurt people and different than identifying other speech that may hurt people and limiting that?
The first amendment as well has "conditions" attached to it. If I am standing in front of a crowd of people with a bull horn claiming know it alls must be killed and the crowd bashes some know it alls head in, I can be sent to jail for complicity in the death of that know it all if it is proven my actions and speech contributed to the death of the know it all.
So if we are going to expand our laws to keep people safe from fire arms, why should we then not expand our laws to keep people safe from hate speech?