Why there is no option of switching sim for sms mid of a conversation ?

Why there is no option given to change sim network for sending an sms in the middle of an on going conversation ? The only way one can switch sim for sending a sms in the middle of a conversation is turning the network off of last used sim (in settings) for that convo, by doing this sms sending gets failed and then it prompts to use other network. What is the meaning/use of setting default sim in settings then ? Apple must add an option of switching sim network in the middle of a convo also ! Its frustrating !!

I dint even notice initially and keep deducting my core balance amount !