Why do weak teeth break or chip?

Fragile. Teeth are usually weak due to decay or previous fillngs. Teeth are the hardest substance in our bodies, but that means they are also more brittle. Think of them like porcelain, a very strong but brittle material.
For various reasons. Trauma, bite discrepancies, bruxism, clenching, tooth decay or too large filling can obviously cause breakage. It can range from small chips to major fractures, you could either need a major procedure or no treatment at all. See your dentist.
Weak structure. As you said -- weak teeth. Either the strength of the tooth structure has been compromised -- large / numerous fillings or decay; or excessive force from parafunctional habits (clenching/grinding), inappropriate diet - biting too hard on very hard or frozen foods/candy. See your Dentist for examination, diagnosis and tx. options. May need occlusal guard after they are restored. Good luck.
Lack of strength. Weak teeth have less tooth structure present. This compromises the tooth leading to it cracking. See your dentist to fix the tooth.