Existing Machinery.
Reasoning about the usage of existing packages
Adding a few hundred transistors for multiplexing is approximately free compared to buying production machinery for several millions - way before the first chip can be made.
Creating a new chip (family) is for sure a risky bet on the future and takes some investment. Keeping this investment down to a minimum reduces the risk taken.
Ordering, building, integration and ramp up of new production equipment is a quite large investment. Requiring this for a new chip might endanger the whole project, as management might not be inclined to spend that money on some fancy and uncertain new stuff.
So calling for new, larger packages not strictly needed to make a new chip would be a rather stupid move for engineers. Especially not if one can come up with schemes like multiplexing to make it work with the existing production environment.
History for larger packages
The driver for DIP 40 package weren't CPUs, but rather memories, as their increasing pincount made 300 mil packages (TTL like DIP) impossible, so the switch for 600 mil was needed. Similar for I/O chips. With the need for devices like 8155 or 6820 as whole families (and the proven success of early CPUs) new machinery for DIP 40 could be justified.
Multiplexing as an advantage
Also, when looking at a whole family like the 8080, multiplexing isn't negative at all. It becomes an advantage. A 8155/56 offers three 8-bit ports and a 256 byte RAM (and a timer) in one single DIP40 package. With non-multiplexed lines, only two ports would have been possible. Similar the 8755 offered 2 KiB EPROM (ROM in the 8355) in addition to two ports. With these chips it was possible to build typical (small, embedded) systems without any demultiplexing at all.
Connecting 8085+8156+8755 was a quite complete setup with 2 KiB EPROM, 256 bytes RAM, 41 bitwise I/O lines and a timer. The only external components needed is a crystal, a diode, one capacitor and three resistors. That's it.
Similarly, the 8088 (and 8086) could enjoy the same benefits due using the same multiplexed bus structure.
So on the whole, multiplexing is an advantage, and only comes with additional cost if one wants to build a maxed out system — which would be a pile of circuitry anyway — so a few demultiplexers won't hurt either.
