They like using the service Google provides and they really want to continue paying for it. Not everyone gets their rocks off serving a popular website and tuning the dozens of arcane knobs to make it hum as well as Google can.
They probably don't want to pay $$$$ for someone else to move it and/or host it. Migrating years of posts and all the comments... that's a decent sized job. Even if the export/import to some other blogging engine was nearly perfect (Hint: it's not all roses and perfume), there's a dozen or more billable hours involved to make it run exactly the way it did before on another host. That's probably more than he spends in a year at the current location.
...and so on. There are plenty of good reasons not to move it.
I don't think he was suggesting switching hosts but domain registrars. Not only does Blogger let you bring your own domain from any registrar while hosting with them, but they stopped selling domain registration themselves at some point in the past. Transferring the domain should be completely transparent to Google; the website doesn't need to move or be changed at all.
