system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do water pipes that bring water to your home never need cleaning?

Basically the fresh water which comes int your home for drinking or bathing. Those pipes can be decades old and still used to bring potable water into homes. That means all the pipes which start at the original water source travel all the way to your home and still is safe to drink. 

I assumed that if their were chemical in the water to clean the pipes it wouldn’t be safe to drink. So how does that work?  It seems like it’s an incredible thing many take for granted.
gpt
The water pipes in your house are always under pressure, meaning that from before you turn your faucet on and during your use of water, the pipe is under the constant pressure of your system (usually 40-70 psi here in US) no dirt or debris gets in them since they are under pressure. And like you said the water coming into them is already potable so no need to clean it. 

What should be cleaned and maintained is your hot water tanks and appliances... hard water (lime and calcium scale) can build up in tanks especially and cause big problems in your plumbing system. It is recommended to flush out your hot water tanks at least once a year to clear it of any debris or sediment.