why are company's like Activision just rebooting video game series instead of starting something new?

Improved graphics, updated/upgraded game engine to better use modern hardwares, maybe add additional content into the game. There could be multiple reasons. I wish that Rockstar Games would properly do something similar with the older Grand Theft Auto episodes. The Grand Theft Auto: The Trilogy – The Definitive Edition is not really that good. Rockstar Games could have been done that so much better.