This comment reads like someone why follows the overarching lore and less the books and the like, which is honestly a very fair take from that perspective.

I think the books do a good job of setting up interesting characters. The issue is that the major players in the Universe are typically not allowed to have any major development in order to move the universe forward for the game. These characters constantly make bad choices . It's a TTG first and foremost and they need a reason for there to be a universe ending threat coming form chaos for that next edition, and they need the Space Marines to save the day last minute to keep the story going.

This means the overarching storyline and the main drivers of the plot tends to be extremely basic as they only exist to move the world forward for the next edition. It's usually the books exploring underdeveloped characters, or the history of the main players, that do some decent development.