Why is there still lag and why is there still people going DC cause of it?

Hi i am here and asking why is there still lag ranked Because my 2 games from last night i have gotten over 8k ping, like really? and it costed me those 2 games in ranked cause i was trying real hard to carry but nah lag wanted to interfere so i went to check my network and see if i have a good connection and it says i do have a good connection and no one else was using my WiFi so i thought it was riot but i guess it wasn't Because i didn't get loss prevented, so like dafuk guys whats going on? and also in both my ranked games there was 1 that Went DC and that would be my ADC and also my support was lagging and my enemy mid laner was also lagging too. So yeah umm is it riot or what?