[sky303]

As an American, I'd say the level of complexity is analogous to the state of the ITV network during the 1990s -- a brand for national programming to unite the network, but strong local identities for news and non-network airtime. (That said, the US will never have a Carlton-Granada merger moment, as FCC regulations on concentration of media ownership, where one station owner can only amass a total reach of 39% of the US population, effectively make it impossible.) Probably the quirkiest parts are the continued use of station call signs (which the UK has never really had) and our approach to channel numbering (there is no centralized set of LCNs, in contrast to UK DTT, and most stations continue to use their analog-era channel numbers for branding purposes).

I never understood why the FCC made this 39% rule, why did they decide to do this? And why the FCC won't allow NBC, CBS, ABC, FOX and the CW to own all of their affiliates

    When TV started in the US it was a bottom up approach essentially.  TV stations would be started by newspapers, like they did in radio, eg: WGN Chicago,  or by business people or organizations who wanted to get into the medium. Early days were also expensive, so there were no major media groups like how Nexstar and Sinclair exist. Until the 1960's besides the networks, there really weren't station group ownerships. (Kind of broad statement, but it would be a rarity, if there were any)   To make sure no one would go too far, the FCC had rules that said you can't own more than 5 stations nationwide, later it was 7 then finally 12.    When ABC and Capital Cities merged in 1985 they had to sell stations to keep it 7.  (Disney currently only owns 8 stations)  The rules were changed in 1999 to a more compromise of ok 39% share.   The UHF discount ( a rule that counts UHF stations as 50% because they had less coverage,  though is mostly outdated and was almost revoked, most TV stations in US are UHF now thanks to it being better for Digital TV)  actually make it seem lower, Nexstar , Sinclair, and probably Gray would be passed  39%.  

   There is a logistical reason,  owning stations in 200 markets (I think the total is 210) would be a logistical nightmare, unless a change to how local TV was done, for news: it'd have to be more regionalized for it work.  There used to be a rule, this was undone just a few years where you had to have some sort of office space , at least, in every era you owned a station.  Ion Television, a small network, owns all their stations (keeping this simple since that's kind of changed now) but doesn't do local news and had a centralized programming line up , and maintained small offices, that'd be harder for a larger network.  
       Also revenues, since the 1990's  networks get paid by their affiliates (used to be the other way around)  so the benefit more by not owning all their stations.   Hope my answer isn't too long.