Because a fleshed-out story and good animation requires a lot of dedication and hard work, which in turn costs a lot of money. In the eyes of the studio execs (and to some extent animators), it's easier to shovel out mountains of half-assed throwaway gags and 1-dimensional plots, than to spend extra time and resources to make something really worthwhile. Teen Titans Go isn't made out of any respect for the original series, it is just a way to make a quick buck by rehashing an older title that has nostalgia value. It's not meant to cater to the original fans, it's meant to cater to the new generation that wasn't actually around for the original series, but has heard of how good it was from their parents and older siblings.