Because in the world of Pixels, they DID exist in 1982.
According to the script of Pixels, the opening scene (set in 1982) has the main characters (as 13 year olds) go the arcade near them, which shows us that in the world of *Pixels, some of the games in question exist before they were released in the real world.
Excerpt from the script (bolded parts mine, [snip] used to indicate I have removed extraneous material);
EXT. SUBURBS, DELAWARE, SUMMER OF 1982 - DAY
Our hero, SAM BRENNER, 13, rides his bike through the suburbs.
WILL COOPER, age 13, is mowing his lawn.
INT. ARCADE - DAY
Its like going into another world. All the classic games are lined up: PAC MAN, DONKEY KONG, CENTIPEDE, SPACE INVADERS, GALAGA, Q-BERT, PAPERBOY, ASTEROIDS, BREAKOUT.
Being as Paperboy exists in 1982 in Pixels when it wasn't released until 1985 in real life, and Q-Bert wasn't released until October in 1982 yet it exists in the Summer of 1982 here, it seems safe to assume that in the world of Pixels, all the games mentioned did exist in 1982.
Either that, or no-one did their homework. You choose.