I've installed apt by rpm (or from a task repo that has no changelogs in pkglist).

The shown changelog is empty:

$ rpm -q apt
apt-0.5.15lorg2-alt90.p10.1.x86_64
$ apt-get changelog apt
Чтение списков пакетов... Завершено
Построение дерева зависимостей... Завершено
Changelog for apt:

$

In apt, some other data is merged from pkglist and from rpm db. The same could be done for the changelogs of installed packages. (And for file lists, and let's have a look what else.)

And it might be not a good idea to take extra space in the memory for this (in the package cache): rpm db can always be queried for this extra info (like changelogs, files, descriptions -- not needed for dependency resolution).