Programmers don't know squat about chess, that's the reason.
Chess.com bugs (not my fault)

Why are expletives, when meant in a jovial manner, met with threats of excommunication? Newsspeak is unacceptable. Be seeing you.

I know, right? And one time when I moved one of my pawns two spaces and it ended up next to one of my opponent's pawns, somehow my pawn disappeared and his ended up right behind where my pawn was sitting! I resigned the game in protest because it was obviously a computer glitch. I wish chess.com programmers would fix these bugs...

The bug I've come across several times is when I've taken all my opponents pieces except for the king and I have several major & minor pieces on the board and all of a sudden the games stops and I only get draw points added to my rating.

This is not a bug, but It happens to me way too often that the system reconnects me. My internet is very good and is not slow at all.
However every single day I have this problem. There is a message that say "Reconnecting", and in the proccess I lose at least 25 seconds in my clock, and for bullet it is absolutely terrible.
I have lost literally dozens of games (In some of them I had huge advantage) because of this bullshit

Don't forget about how pieces somehow teleport to the opposite side of the board some times! This is extremely annoying!!!! Expecially when the clock is ticking and you're trying to calculate KP endings.
Then the next time my opponent moves it...It's back where it was supposed to be.
I pay for this membership..I at LEAST expect the pieces to land where they are placed.
It seems to be mainly an f3 and c6 phenomenon.
A lot of people have been complaining about all the bugs with chess.com and I thought I would mention some of the really irritating ones that chess.com has not fixed in the 5 years I have been here.
First of all, there is this weird thing where sometimes my opponents' pawns move two squares, when everybody knows that pawns can only move one square. I wonder if the chess.com programmers even read the basics of how the pieces move. This is by far the most common bug, although not the most frustrating.
The second problem I get is that sometimes I can move two pieces at once! This obviously isn't right. It seems to only happen with the king and rook, and only some of the time (usually the king hasn't moved, and even then it doesn't always happen). I haven't noticed it with other pieces. It's a pretty small bug, but still bizarre and against the chess rules, somebody should look into it.
Third, and most frustrating of all, sometimes I am way ahead on material but chess.com says I am checkmated? WTF is that?! The rules, as I presume based on no research or knowledge on my part, clearly state that you cannot lose if you are down on material. This is plainly unfair and I hope chess.com fixes these bugs immediately!