why is forcing a draw still part of chess?

Ok so i was playing a blitz game and i was COMPLETELY winning with 2 rooks and 1 queen and the opponent only has his rook and queen left but suddenly he kept checking my king and suddenly the game draws and that just annoyed me so much because in like 3 or more moves he's already mated so here's the main question, why is draw by repetition even in the game? what is the true meaning of forcing a draw? doesnt that just make you look like a coward who was a phobia of losing? i cant believe they even have the will to do that
please correct me if im wrong and if my understanding of draw by repetition please explain what it really means

I do understand...it's annoying to get yourself into a draw trap despite having a winning position. Now why is 3 fold repetition is considered draw? It is so because repetition only leads waste of move, time and game. Understand it in this way.....

In this game black has no choice other than keep checking the white king. If there is no such repetition draw rule then the game will go forever....Now why not consider win for white? It is because game is not ended yet although the white has piece favor but can't make a use out of it unless the repetition breaks. All the move by black is legal and white can't escape the forever trap. So to make this reasonable and simple... force draw by 3 fold repetition is a rule. And it is a key tactic to draw a lost game.  

I've heard somewhere that once someone proposed a theory that king can't be checked more than 6 times in a row so to avoid the repetition draw thing...but this will only increase complications related with certain mate patterns such as rolling rook mate and repetition not necessarily include checks. 

Thanks but still its annoying so this means i just gotta upgrade my defense on winning postiton end games?

Nah you don't really need to build defense in end games, play normally as you do  just make sure your king can escape or can be protected every time under checks. Lasker once said "The Hardest Game to Win is a Won Game"

In a won position always look out for stalemate or perpetual check.

^^

Yes please

"i was COMPLETELY winning"

This is a common delusion! If you were actually "COMPLETELY winning" your opponent couldn't have forced a draw through perpetual check. You just THOUGHT you were winning! 

Important when you are playing for a team in a match, your captain is allowed to advise you to go for or take a draw without giving an assessment of the position. Sometimes you sacrifice half a point for the sake of the match.

It means you have to actually find the winning move when you have a "completely winning" position.  The threefold repetition rule gives you at least two chances to find a winning move instead of the move that leads to repetition.  If you can't find the winning move in two tries, then you probably don't actually have a "completely winning" position.

In particular, if your opponent can actually force a draw, then it's a drawn position, not a winning position.