I'm not sure that is the primary reason -- many other projects have had their compile times explode over many years, even though the same logic should apply.
Not to mention if you build the kernel regularly, you benefit from incremental compilation. If you change a few non-header files the rebuild time can be as little as 2-3 minutes. Oh, and "make localdefconfig" will reduce your from-scratch compile times to the 5-15 minute mark. I highly doubt most kernel devs are building a distro configuration when testing (especially since they'd be testing either in a VM or on their local machine).
When I worked at Microsoft, Windows took far, far longer than an hour to build from scratch. I remember walking a few buildings over to the burn lab to pick up DVDs of the latest build. I don’t have any hard data, but running a full build on your dev box was very rarely done.
The dynamics of commercial projects can be very different from open-source. You're much more likely to tolerate sitting through long builds if you're being paid to do so.
Not to mention if you build the kernel regularly, you benefit from incremental compilation. If you change a few non-header files the rebuild time can be as little as 2-3 minutes. Oh, and "make localdefconfig" will reduce your from-scratch compile times to the 5-15 minute mark. I highly doubt most kernel devs are building a distro configuration when testing (especially since they'd be testing either in a VM or on their local machine).