the time is right for him to go to the next level. He has had a really good season, after the bowl game he'll be over 900 yards receiving.
Could he improve his numbers next season if he stayed? Probably, but he can also get injured // banged up and miss time. It's a risk. He has had two healthy seasons in a row adding durability to his resume.
Even with his numbers not being elite, NFL scouts know what he brings to the table, a reliable, durable and mostly surehanded receiver who has size to make contested catches and a knack for making catches when they are most needed.
I believe Chase will surprise people in the draft, he's going to go sooner than people assume.