Serious question, why is Tryndamere allowed to ult through every type of CC in the game?

Probably because his theme is hitting his ult as late as he can before dying, to cockblock death... not running into a fight and having to decide between "let's not ult and get instadeleted" and "let's ult before the fight starts and then laugh bitterly when the the ult gives him absolutely zero benefits".

Not all, silences prevent him from ulting.

Because trash players wouldn't be able to reach gold.

'Because playing Tryndamere is hard.' - Riot Games, who totally have no bias when the champion is literally named after one of them.

Also he got buffs.

It's fucking stupid that he can press R in the middle of an Urgot ult that is a SUPPRESSION. He needs to be reworked so that cancerous ultimate of his can be removed, replaced, or updated to make sense in the scope of all other rules the game operates under.