Jörg,
Because it still means we have to fight all the breakage it creates for very small gains. It is a question of maintainance.
Granted for maintenance. But NetBSD cannot stall forever on a increasingly obsolete GCC version. gcc 4.1, for example, has no specific (integer and schedule) support for Core 2 architectures, if I am not mistaken, this has been introduced in gcc 4.3. Now, there are the new Penryn processor out there, with sse4, supported only in gcc 4.3. Etc.
Vincent