Why is it so rare to see a unseeded tennis player win a grand slam?
To raise a grand slam trophy, a player has to win 7 consecutive best of 5 set matches and in all likelihood against several top 10 players. Tennis is considered to be a demanding sport both physically and mentally. The players have to be in top shape and be ready to duel out many tough 5 set matches during the course of the slam.  During grand slam matches, players can run anywhere from 3 to 4 miles with most of the movement being lateral. The top players have the distinct advantage that they have large support staff including dedicated physios and massage therapists who can get the player ready for subsequent matches. Besides, the top players have coaches who may have themselves won slams, and thus carry a wealth of experience. Sometimes players also enlist the services of data analysts who can do opponent video/data analysis to determine patterns/preferences of opponents. Such massive support staff carry a lot of payroll that most unseeded players cannot afford. For this reason, though at times we see a qualifier make it through to later stages of a slam, they will rarely ever get past quarter finals let alone win a grand slam.