They can't, the RAM was external which wouldn't fit on the one sided Zero. Thus why they bounded the 512MB ram in the package for Zero 2. Physical limits are

They can't, the RAM was external which wouldn't fit on the one sided Zero. Thus why they bounded the 512MB ram in the package for Zero 2. Physical limits are now everything you'll see until the end of time. 8GB is the largest RAM you will ever get in one chip. 1TB is the largest Micro SD. Etc.