If they made single-chip Z80 computers, how many would you buy?
They do, you know. Zilog continued along and did microcontroller versions of the z80 with on-chip memory and peripherals.
It looks like the top-of-the-line runs single-cycle at 50MHz, has 256k flash, 8k ram, and various peripherals including ethernet.
Also, NEC did enhanced Z80 things, before becoming part of Renesas, and some of the Renesas CPUs are rather depressingly z80 like. Including the recently hyped RL78 series...