Major telcos are anywhere from 70 to 100-year-old companies generally speaking. Most of them are the result of haphazard mergers between many many local phone companies over their existence, so even internally they are often held together rather precariously.
I'll give you one simple example of the magnitude of this. In Ontario & Quebec, Canada, in the 1920s, there were almost 800 local phone companies operating, and "The Bell Telephone Company of Canada" was the long-distance provider that connected all those companies together. Even back then they handled almost 3M long distance calls per day (from roughly ~500,000 phones).
Over time, Bell bought up all those local independent companies and merged their records, customers, infrastructure, operations, etc..
That's Ontario and Quebec only, 2 provinces out of 10.
Fast forward like 60 years and in Canada local/regional phone companies in various parts of the country were still a thing in the early 1980s, and even now we still have distinct phone companies for some of our provinces.
And this is just one country that has less than 40M people. Now repeat this process in the US, and other parts of the world, going back almost a century, and you can start to understand the complexity we're dealing with here.
The insanity of these merged and glued-together tech stacks would make most people faint.
Obviously they're not all still running on super old tech, but if you look at any major incumbent telco's DCs you will commonly find switching systems from as far back as the 1960s that have been wrapped in layer after layer of "modernization" but are still there routing calls and running old code.
I know you believe what you are saying is "simple enough", and it probably should be, but sadly it's not.
And while you're right to say it's a cost thing, it is also a technical problem in that it would require massive coordination both internally within telcos but also between companies that are competitors to each other, and aren't naturally inclined to work together in the first place.