Why is it possible to start a game while the time is still running in another game?

My opponent stopped playing when he saw that he would lose all of his material, including the queen and both rooks, and when I hovered the mouse over the player's name I could see his new ongoing game, that's much worse than just abandoning the game, he abandoned our game and STARTED ANOTHER!!! How does the server let this happen?

Lichess should not allow to start a new game when another game is still in progress, except correspondence and simultaneous displays.

Indeed, that was exactly my proposal. Playing simultaneously is a no-go at other places. If you play Blitz, there's no need to start other games, is it? Respect is another story.

"One should separate single game mode (Bullet, Blitz, Rapid) and sim mode (Corr, Simultaneous exhibitions)."

Sometimes i play more than 1 rapid or classical games at once, i don't think there is nothing wrong with it.

It is disrespectful towards your opponents.
Lichess should not allow it.

Come on seriously? I have read from #2 link the discussion in the other questions @tpr said that playing more than one game "is insulting towards the opponents" due to you are not focus on his game. So tpr, i suggest to ban music from internet because it can distract my opponent, oh well an other improvement could be that the app may not work if someone is moving, if he walks he could not be 100% focus on the game. Really?
Sorry but i really don't understand your point, i have my time and i play at best of my possibilities, for me not for you. If you pretend that your opponent is sitting at the table just thinking at the position well you should go in a club...but for long games they also can stand up and walk xD

Sure. Lichess, please stop this polygamy.

#7 You can listen to music, watch videos while playing. Over the board you can walk, you can study paintings on the wall, discuss with other people everything but the game in progress.
However, no tournament organizer will let you play 2 games at the same time. Lichess as online tournament organizer should disallow this too. First finish your ongoing game, only then start a new game.
You cannot lure people into a simultaneous exhibition without it being known to them.

Why so much complains about people playing multiple games? That's not disrespectful. The good news is they'll be less focus on your game ergo you have more chances to win them. Honestly, those people won't achieve good results by doing that, so stop complaining about everything.