Because if he blew up this year he'd bolt. They did exactly what Charlotte did with Lamb, though Lamb has been even less proven in his NBA career to date (which is why he signed for less). They gambled on his development and a potential breakout coming off the bench as a main offensive option, or if not a stall and later breakout once DD leaves (taking on the starting SG spot). There's still plenty of time for him to pick up his game, but it's obvious we're seeing a lot of what he did for the past three years. At worst he is still a tradable piece because the salary cap is going up.
That's not really panicking though. Panicking would be the Thunder matching the 4/70 Portland threw at Kanter. If Ross did happen to breakout and Masai rewarded him for it with a big extension, then that'd be panicking.