They don't, they atrophy, which is different. Treatments can potentially induce healthy hair growth once more if all the factors are addressed.
Male Pattern Balding May Be Due to Stem Cell Inactivation. The authors concluded that totally bald scalps do not have fewer hair follicle stem cells, but rather, fewer specialized progenitor cells.
http://www.uphs.upenn.edu/news/News_Releases/2011/01/male-pattern-balding-stem-cell-inactivation/
Effect of scalp burns on common male pattern baldness. Case study in which they discussed an old man who burnt his scalp badly and then regrew a lot of hair in the burnt region that had been missing for decades.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1351889/
Reversal of male-pattern baldness, hypertrichosis, and accelerated hair and nail growth in patients receiving benoxaprofen. Benaxoprofen regrew hair in a 75 year old patient who had been suffering from androgenic alopecia for 30 years.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1498062/?page=1
Topical Diclofenac 3% Gel for Actinic Keratosis May Induce Terminal Hair in Male Androgenetic Alopecia: A Report of Three Cases. A study from 2015 fund that topical Diclofenac 3% gel for treating actinic keratosis (also called “solar keratosis”) resulted in three elderly patients regrowing some long-lost hair.
http://journalssc.com/?page=article&article_id=33470