Various gnome-shell running after gnome-shell terminal closed

I start from gnome-shell to run the following command

tmux new -s "main" alacritty & 

Then run commands on the new shell of alacritty

pkill -9 terminal #to kill the gnome-shell terminal  

At the moment, pstree to review the current process

me@host:~: $   ps -ef | grep shell me    2986  2831  4 12:08 tty2     00:00:06 /usr/bin/gnome-shell me    3049  2745  0 12:08 ?        00:00:00 /usr/lib/gnome-shell/gnome-shell-calendar-server me    4073  4020  0 12:10 pts/2    00:00:00 grep --color=auto shell me@host:~: $   pstree -asp 2986 systemd,1 splash   └─gdm3,1710       └─gdm-session-wor,2695           └─gdm-x-session,2816 --run-script...               └─gnome-session-b,2831 --session=ubuntu                   └─gnome-shell,2986                       ├─ibus-daemon,3020 --xim --panel disable                       │   ├─ibus-dconf,3024                       │   │   ├─{ibus-dconf},3027                       │   │   ├─{ibus-dconf},3028                       │   │   └─{ibus-dconf},3043                       │   ├─ibus-engine-sim,3401                       │   │   ├─{ibus-engine-sim},3402                       │   │   └─{ibus-engine-sim},3403                       │   ├─ibus-extension-,3025                       │   │   ├─{ibus-extension-},3035                       │   │   ├─{ibus-extension-},3036                       │   │   └─{ibus-extension-},3047                       │   ├─{ibus-daemon},3021                       │   └─{ibus-daemon},3022                       ├─{gnome-shell},2988                       ├─{gnome-shell},2990                       ├─{gnome-shell},2991                       ├─{gnome-shell},2992                       ├─{gnome-shell},3007                       ├─{gnome-shell},3014                       ├─{gnome-shell},3015                       ├─{gnome-shell},3016                       └─{gnome-shell},3017 

Why are there still so many gnome-shell running?