Activity Monitor does not allow you select multiple processes to kill them simultaneously. You would need a different application to take that design goal into account and implement it.
The only exception would be if your selection happened to be all part of the same process tree where you could kill the parent process and expect all children to receive the termination signal. But that’s how the OS works and not about Activity Monitor’s design.
Now, workarounds to do what you ask in other ways:
Killing Multiple Processes with same name
Using Terminal, you can kill several processes at once:
pgrep -i <process name> | xargs kill
This has the benefit of killing all the processes of, say, Firefox (I have multiple instances of Firefox running all the time) and can kill them with one command.
-i flag tells
pgrep to ignore case so, "Firefox" = "firefox" = "fireFox", etc.
Killing Multiple Different Processes
Use the OR operator to kill multiple processes with different names:
pgrep -i "<process name 1>|<process name 2>" | xargs kill
It's very, very rare that all the identically named, but discrete process have hung up simultaneously, so I prefer to address each on individually to see where the issue manifests. So, I don't recommend it as a regular "go to", but it will get the job done.