Basically because the VM wish to maximise revenue. Also the way systems are set, the accounts usually require manual manipulation to make changes. there is also the way bundles are set up to take into consideration, as downgrading/removing parts of a package can effect discounts.
There can also be issues such as a customer having a V6 box which needs a VM broadband connection, if such a customer removed the VM broadband from their package then the TV part would also cease unless the customer changed to the old TIVO box which doesn't support 4k and all the currently avaIlable apps on the V6 box etc.
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali