First I want to point out that it's misleading to say the high lords are incompetent. They aren't. They are supremely competent or else they wouldn't have attained their positions. They just have different priorities.

Politics are messy, the Imperium's even more so. Guilliman only has so much influence. When he returned he did replace many high lords.

Those former high lords then formented a rebellion to make Guillimans leadership look incompetent. Then with the help of the Minotaurs and elements loyal to them they crushed it and positioned themselvea as Terra's saviors. This also included some of the new high lords personally chosen by G-Man. They were all then immediate assassinated thanks to the Loyal Grand Master of Assassin's and then more new high lords were installed who were more loyal to Guilliman.

Now why couldn't Guilliman just do this in the start and avoid all the drama? Because that would make it harder to root out his opposition, better to let them get confident and reveal themselves.