Unfortunately this is quite common with this type of game developed by one person. The reason is that commissioning art for all the many scenes this has is expensive, especially when going for a consistent style.
Adding art to this game is a goal for the future though if it becomes feasible to do so. I estimate at the low end, this costing around $3,000. As a solo developer who is doing this as a hobby, that's not the sort of expenditure I can make at this time.