licensing – Why do projects keep around old incompatible licenses?

I’ve noticed that in a couple of fairly large repos (GNU Autoconf and the Linux kernel), there are multiple licenses provided even though they’re incompatible.

In the GNU Autoconf repo, they provide both COPYING and COPYINGv3, which are the GPLv2 and GPLv3 respectively. These licenses are incompatible, and file copyright headers reference v3, so I think it’s fair to say that Autoconf is released under v3 exclusively.

My question is: why do projects do this? Couldn’t they just replace the old license with the new one and keep things cleaner?