There are two main contributing factors to why your garage gets hot and steamy on a summer day, which are lack of ventilation and lack of insulation. High humidity levels in poorly ventilated spaces can raise indoor temperatures which can cause the air to hold more humidity until the air gets too saturated with moisture leading to that stuffy feeling in a room.
Garages are spaces for vehicles and home storage and, as a result, they may lack insulation. Insulation blocks heat from entering your space in the summer and holds heat during the winter which results in a cooler and more comfortable space during the summer and winter months leading to lower energy bills. Other contributing factors can include the direction your garage faces, the color of your garage door, and storing a recently used vehicle.