Why do i have to watch LCK SKT1 vs JAG on youtube and not on LoL website?

Use Twitch

You need to check/click the leagues you wanna watch at schedule page on the right side, once you click em it will light up and show all the games of that particular league you chose at the left side of the page plus the ones that were already selected, you can select only the ones you like or all of them, if you want. also don't use twitch directly but through lolesports or it won't count for the game watch streams missions.