Certified GKH Gardening Expert
They can curl due to many reasons, including genetic. Most of the time, leaf curling is is completely normal, however, it can indicate that the pH is off. You should aim for a pH of about 6.5 to 6.9 in the soil. This will ensure that the only leaf curl is a natural leaf curl.
If you are noticing any other symptoms other than curling leaves, then it can be a sign of leaf curl virus, but in most cases this is just a defense mechanism of the tomato plant.
This article will give you more information: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-leaves-curling.htm