Anyone understand why mobile phone operators can't let us use the same account and phone number on more than one sim Card...??

I have an account with a mobile operator that I am reasonably satisfied with,
and 3 smart phones still in good working order, each more useful than the others for a specific purpose.

It would be very useful If we could have a Sim Card in more than one phone tied to the same phone number and account.
Surely the big mobile providers could adopt technology that effectively records data usage over over a reasonable amount of Sim Cards
within the same account...???

It's such a pain having to use different pay as you go sim cards and numbers, alongside my main monthly account.
and not practical to keep swapping the card between phones...

It can't be that difficult for the phone companies to sort out this very practical solution...????