Smiddy
The only reason I can think of is that all everything created by God has its own purpose. Angels have been used by God for various purposes, executioners (for Assyrians), transmitting messages (Gabriel), rescuing individuals (Daniel’s three friends, Peter, Lot), encouraging (Jesus in the garden), and performing ministering work (Heb. 1:14), etc. The last time they did something against God ended badly (during Noah’s flood)
In the Bible, angels have never been used in rulership positions for earthly matters. Even in the greater arrangement of things, they have not been purposed to be used as rulers or priests. In fact, God chose to reveal secrets to men on this earth, not to angels (1 Peter 1:12).
And observing will always be observing. Angels will have a wealth of insight and information into human behavior through their years of observing. But the most important aspect that they lack is mental and physical involvement and exposure to varying human patterns. Centuries of observing from heaven will never give them the physical, mental, emotional, spiritual, religious, social, subjective and tacit experience that can be gained by just 1 year of physical involvement with humans. Jesus experienced all of this as a human.
And so I feel humans will always be apt for the job.
But yep, will feel sorry for the pissed off angels.