I had my wisdom teeth removed 13 days ago and the teethnext to the extraction site, on the bottom left, are still sore, not the extraction site though. Why could it be?

Not uncommon. Patients who have wisdom teeth and /or other teeth extracted may have some sensitivity to hot and cold foods or liquids after surgery. Some patients may also feel that their teeth or bite feels a little different after surgery as well. The sensitivity and soreness usually resolves within a short period after surgery.
This is normal. Is the soreness a throbbing or aching pain or sharp? Is it when you bite down or is it always there? Getting worse, staying the same, or getting better? How long did the extraction take? These are all questions that can help determine the cause of the ache. There was probably pressure placed in the general area and the soreness you feel is most likely normal. Have your dentist do the post op check.
Healing. Healing after removal of wisdom teeth, especially if they were impacted, can take some time. Some teeth may have been more difficult to extract causing delayed healing and complications like trama to nearby teeth. These teeth may have become sensitive or the sockets bruised allowing for soreness like to chewing. Your dentist can reassure you of any problems but it may take some time.