For one you might not always have a mobile version of every page of a gigantic system. Some things might be extremely hard to represent on a mobile device without a lot of rework. If you start talking about months, or years, sometimes to redevelop a complex system..then you can start to imagine why shortcuts are often taken.
Changing domains can be a security nightmare especially with ssl and cookies. Start working on something that must meet various industry security standards and pass code audits and penetration tests and what seems so simple on the surface becomes a deep issue. Some of these legacy systems might have complicated url structures with encrypted data in them that is valid one-time-use.
Basically your simple suggestion isn't going to account for proper handling and carrying of sessions across those domains on a SSL connection. Often times you end up having to rebuild the sessions between domains while figuring out how to do that in a secure manner that isn't about to get hi-jacked.
I'm not making excuses Richard. I'm stating that you are making statements like fixing these problems is just one line of PHP. If you could fix all these sort of problems with gigantic web applications that have a lot going on with one line of PHP you'd be a very rich man.
Much of these problems are intended to be solved by thinking about mobile first, doing responsive web design, etc. Pretty easy to do starting from scratch...can take years and years to evolve to that point with something in production with hundreds of thousands of users.
