Why are 32bit SteamIDs always reported as STEAM_0:*

What? This sounds like a question for Valve - We (SteamRep) are not Valve and are not affiliated in anyway.

Simple google search would answer your question: https://developer.valvesoftware.com/wiki/SteamID

ok

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Well it explains the different universes and what they mean. If that's not what you were asking then sorry..but Valve would answer your question 100%

It really doesn't matter, do you honestly need to know what universe a users account belongs to? It makes no difference in identifying them.

Like, searching any of these on SteamRep will return the same person:
STEAM_0:1:52569926
STEAM_1:1:52569926
STEAM_2:1:52569926
STEAM_3:1:52569926
STEAM_4:1:52569926
STEAM_5:1:52569926

Majority of simple converter functions you'll find ignore calculating the universe ID. For example, the one you generally find when Googling (note the first line of the function):

PHP:

function convert64to32($steam_cid)
{
    $id = array('STEAM_0'); //This line, doesn't even both finding the universe, just sets it at 0
    //was another line here, but cloudflare was chucking security errors at it, removed it for this example
    $id[2] = bcsub($steam_cid, '76561197960265728');
    if(bccomp($id[2], '0') != 1)return false;
    $id[2] = bcsub($id[2], $id[1]);
    list($id[2], ) = explode('.', bcdiv($id[2], 2), 2);
    return implode(':', $id);
}

So that's probably "why".

I haven't yet glanced at the code used-- inherited from before I joined the site, but it's almost certainly what Enstage mentions. Probably more "correct" to fix that but wouldn't be a high priority (and playing with age-old behaviors like that sometimes has odd consequences).