The bottom portion of your garage door is most susceptable to this kind of wear which can lead to water damage and wood rot.
Garage door exterior surface cracked.
Loss of caulk s initial adhesion and flexibility causing painted caulk to crack and or pull away.
You are more apt to see these types of cracks on a garage floor that has a hard smooth troweled surface than one that has a broom finish.
The surface is also easy to clean but the door itself may break if you hit it with a car or tractor.
Gravel and or sand on top of the sub grade.
Caulk paint cracking or crawling the cracking of the paint film over a caulked area.
Repairing the doors requires a little work and depending on how large the crack is could be.
If the garage door panels are wood then yes a garage door panel that is cracked could be fixed using wood fillers and glue.
If your garage door is metal or vinyl it is much more difficult and even impossible to fix.