Why isn't every Imperial university science and engineering graduate educated to TL15?
Why doesn't the Imperium promote the building of TL15 infrastructure on every member world?
Why does the Imperium not insist on universal sophont rights?
Why does the Imperium appoint nobles rather than allow worlds to elect their subsector rulers?
Because The Imperium does not own or control worlds.It is made cleat in the OTU that the Imperium owns the space between worlds.
So,
The Imperium spends the funds it has controlling trade, maintaining defense(and Offense) and diplomacy.
I know many believe, "there has to be more than enough for that" and never see the true bills
....And never see the systems withing organizations where the Government is "cheated" so the service can do what is needed
(I saw the innards of one such effort where the Pentagon was cheating the US Government.....It was an education)
So, the Imperium can't just grab land and build facilities.
*I See* Imperial reserves on member worlds as territory granted by that system's government.
Even the "Imperial" Starport is a location, IMTU, where a system makes deals with one or more corporations to build such a facility and then build or hire(As in the vaunted "Starport Authority" - which is NOT an Imperial Government organization) an organization to control it.
The only "authority" the Imperium has over that facility would be, IMO, Confirmation they would follow Imperial guidelines in order to have access to Imperially-owned space.
So, where the Imperium can demand "No Slavery", they can't demand universal sophont rights, or enforce "any laws" where they hold only diplomatic authority at best. To intervene would be to declare an Imperial Interdiction. Which is a military action.
And finally, the Imperium appoints nobles to represent Worlds, Solar Systems, Subsectors and more because the Imperial Government then - According to the OTU - grants that nobleperson a "slice of the Credits" from that location's taxes. That money is "expected" to be used to encourage and invest in improvements to that Fief.
So, Leonard, Duke of Rhylanor gets:
1) A slice off the world Rhylanor, and the lesser worlds within the Duchy which do not have a Baron(ess), Marquis(a) or Knight/Dame assigned directly.
2) A lesser "over all" slice of the trade taxes drawn from the Duchy of Rhylanor
3) An amount of any funds granted him by Grand Duchess Delphine(1105-era) of Mora from her funds as that Nobleperson who holds the Spinward Marches Sector as her fief.
Because this tax money - From the Imperial Tax Purse - is granted to each member of the Nobility appointed to hold a world )or even nation state where said world is balkanized) in fief, --- because of that - the person is appointed. Not elected