system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't our bodies limitlessly produce vital cells such as brain cells but we can practically infinitely produce more or less useless cell types like body hair?
gpt
One thing to keep in mind is that your brain cells function in a complex network whereas dermal cells can basically be dumped into layers in large numbers without affecting function. They just have to be there and that's enough. Neurons are unique from all other cell types in that they have much more to do in terms of function and networking. 

For this reason your brain and body actively inhibits cell growth in these network areas such as the central nervous system once the networks are in place and working properly. 

When you're an infant your brain allows these networks to grow much more freely so that they can figure out how to interpret the world. Indeed neurons *can* proliferate rapidly - we see this through embryological development and infancy, they just slow down significantly as an organism matures. Once they start firing reliably you prune away (i.e. get rid of) the less active ones and then change the chemical milieu to something that inhibits growth more. 

Specifically why this is the case >> It's bad for your entire brain to constantly be in growth mode. 

The short version is that both brain cells and skin cells are reproduced by a progenitor cell population that stays in an undifferentiated constantly dividing state. In the skin, you can send the daughter cells away from this progenitor level and as they make it to the surface they'll wither up and die (surface-of-your-skin cells are actually dead, which shows how they're fundamentally disposable) and be perfect for wherever they are. They'll easily replace damaged or sloughed off cells from previous generations. 

In the brain, you'll have a neuron that has hundreds of connections - some local, some extremely far away, each specifically nourished by different processes (glia were mentioned in another reply) and each having survived various neural-pruning events. If this neuron dies - how can it's replacement mimic it? Travel from the basal layer, find the "missing space", differentiate it from other missing spaces that simply exist or were created due to pruning events, and then start participating in a complex network? It can't just travel there and succeed by virtue of being there. It needs to make all those connections - but all those connections formed through the organism's *experience*. They can't just be programmed.

So lets assume that a new neuron can somehow figure out how to get there - a hypothetical biological alarm can allow the new cells to find exactly where the lost ones lived. It sends out growth cones to make connections. What happens if these wire up to the wrong targets? Your existing, successful networks (skills, *memories*, etc) are now screwed up. That's not good.

Thus we don't want this neuron growth constantly happening in our brains because for the 99.9999% of mature organisms that aren't actively experiencing brain injury, brain growth will actually be messing with a good thing, "fixing" something that isn't broken. 

There is a molecular background to this that is more detailed and not fully understood but this essentially is a major difference in the two processes.