>>52917708They have the same ideal moveset, and since physical vs. special stats don't matter in GO, the fact that Chesnaught has biggest Attack and Defense than Venusaur has Sp. Attack and Sp. Defense (plus more HP) means it has better stats in every category. Also, Sceptile is more frail without significantly more DPS to consider it better than Chesnaught. Don't forget that Mega Venusaur and Mega Sceptile exist, though, along with Shadow Venusaur, Shadow Tangrowth, Kartana, etc.
