The Telco's prime motivations are to make it easy for their customers to make more calls and send more text messages - that's what make them money. Making it difficult to port phone numbers away from competitors, or making it difficult for customers to redirect phone calls or text messages is bad for business, and will lose them customers and/or increase customer support costs.

There's absolutely no upside to a Telco for doing any of that - they don't really care if that makes their product less secure for use-cases other 3rd parties who aren't paying the Telco - the Telco's never signed up to provide a secure channel for your bank to send secrets to you - and the banks aren't offering to pay them for it. The Telco's customers are paying, and to an overwhelming degree they demonstrate that they prefer convenience over security - you get pissed off and potentially change carriers very quickly if you cant call them up from a friend's phone when yours get broken/stolen and get your calls/texts redirected to another number immediately. The fact that that same ability give access to social engineers to get hold of banking pins and internet service 2fa secrets is a vanishingly small concern for the people paying the Telco's.

So they just don't care. Not their problem. Sucks to be a bank.

Not that I think we'll be rid of SS7 even a decade from now, but there is a reasonable path to take, yes ?