Obviously it does depend on their contract, but since a couple of members have said they did consider opening up individual accounts in the past. Therefore, I am certain JYPE is not stopping them from opening up solo accounts if they wish to do so.
The reason why they don't have individual Instagram accounts is probably for the same reason why they choose not to do any solo debuts yet, which Jihyo stated was so that they can all remain as one and focus on the group as a whole instead of just themselves.
Furthermore, Twice is one of the biggest k-pop groups in the whole world, with a couple of members having issues regarding anxiety in the past couple of years. Opening up individual accounts will have some benefits and feel more personal, but also remember it has a lot of downsides, such as extremely toxic haters who will send them hateful dm's/comments. Also, some members will be followed more than other members, which somewhat reveals solo stans, and I doubt the members of twice would want to know the difference in popularity among themselves.
