You don't just "add them on". They have to be jigged to be properly aligned with the eventual gas tube hole in the upper receiver (aligned with the nub on the top barrel extension)
Then the barrel has to be bored out properly to receive the taper pins in alignment with the FSB. That's gun smith work. So in Canada that translates to 120 an hour and a 5 month minimum wait period. Also a decent FSB is far more than $30. A DPMS undrilled block is $55 CAD before tax and shipping. There is ONE place selling them advertised in Canada right now. Maybe $30 on the EE if you can find one.
Ask me how I know all this! I have a 20" 1:9 pencil barrel at Tier One right now getting this very thing done. I had to buy a complete Norc DA556 Upper just to take the barrel off it.
The thing is it's way easier to drift the taper pins out and take off a properly made FSB than it is to add one. So if people want to build some movie prop john wick gun they can easily do it in their basement while drinking a soy latte from Starbucks.
Us old fashioned dinosaurs who like to shoot irons and don't like critical components of our rifles held on with set screws and lock tite can't do proper builds which sucks