Because they don't.Yall act like rockstar doesnt always take years to make a new game.
Gta iv came out in 2008. Gta V came out in 2013.
In between 2008 and 2013 they released the lost and the dammed, Chinatown wars,the ballad of gay tony,midnight club Los Angeles, max Payne 3,beaters tor and red dead redemption and RDR undead nightmare.
Between gta 3 and iv they released 2 other GTA's, bully,manhunt 1 and 2,smugglers run,multiple midnight club ga,the warriors,state of emergency etc.
Between 2013 and now they haven't announced a new game,yet alone released one.
This is unprecedented in the company history since gta 3.