One of the features of the Z80 was built-in memory refresh, so it needed a lot less glue logic than most processors, which had to have external registers and latches to handle the refresh cycles.

Way back I used to hand solder Z80 embedded controller board prototypes, and they never needed much in the way of chippery.

In an S100 system the biggest extras were the video (formerly VDU) cards, and the memory cards.

Both were limited by the RAM of the day, because RAM came in tiny sizes like 1 bit x 16k dynamic or 2k x 8 static. So you needed a lot of chips on a separate board for what was really not much RAM at all.

Also, money. 64k boards cost over $1000, which is > $3k in modern money.

Now you can get 8 bit x 256k static RAM on a single chip, so you can park that next to the processor without a separate card. With a bit of glue you can even share it with the graphics hardware, eliminating any need for separate graphics RAM.