Just did the math and supporting a ~70F (50F-> 120F) rise at 1GPM (faucets are limited to about this) requires 9kW, or about 40A at 240W. Supporting a bunch of 40A circuits in a typical home would require a massively upgraded circuit panel for most homes -- 200A or even 100A service is common.

A centralized system avoids having multiple circuits with potentially high loads like this. Not to mention the likely gigantic heat exchangers required to transfer 9kW!