The reason for not using the framework is
I think that the following thoughts may be made (aside from the correctness).

① If you don't know how long it will be used, it's better to have no framework
(2) Consider whether to import each time the framework version is updated + refurbishment + testing occurs
③ If you're testing all branches, you can't figure out what the framework is doing

I think that it is the feeling of the IT person on the customer's side that they basically don't want to touch it once it is released.

People who don't understand the framework won't be involved in related work,
It is necessary and costly to provide the same level of education to those involved in related work.
May conflict with mechanisms already in use,
It takes time to migrate when introducing a new FW

I think there are many.

That's something you should ask the customer

Probably to avoid blaming the framework for functional limitations?
Requests made by customers

I can't do it because there is no function in the framework

Implementation because it does not match the framework specifications

This is because the development side is willing to talk about things
Contrary, let us know that there is a cost for creating each function without using the framework functions
You will have to choose the price difference based on the estimated price

It is also possible that version management after installation will be done at the customer's responsibility
I'm a little worried, but the framework, greater or lesser, is such a thing
I think it's part of the job to explain it properly and be convinced