Tons of other processes are still causing load even when Apache is stopped, such as: Plesk (which runs tons of stuff in the background), MySQL, PostgreSQL, mail server, system tasks, NodeJS apps and long-running Python apps on people's accounts, cron jobs, etc.
Apache itself is only a fraction of the load, and while yes, load will drop when Apache stops because there's no requests (which means things like PHP and CGI stuff will not execute), it doesn't make much difference. The real determining factor is the number of vhosts in the config. Apache takes its sweet old time loading all those.
It's worth noting that Johnny is really overcrowded for a Plesk server. Typical Plesk servers are meant to handle ~400 accounts per server. We have 991 on Johnny right now, and it was over 1000 a few days ago (several accounts archived and caused it to drop). We've actually been told by the software companies (both Plesk and cPanel before it) that they were amazed at what we've done with their products.
Morty is much faster than Johnny for these restarts. There's substantially fewer users, fewer vhosts, and its running on hardware that's both newer and faster than what Johnny is on. It's been a while since I checked the numbers, but I remember Johnny's Apache restarts were like 5-10 minutes while Morty took just 9 seconds...
TL;DR: Server has a lot more running than Apache, and is overcrowded.