When you source, you’re loading the activate script into your active shell.
When you do it in a script, you load it into that shell which exits when your script finishes and you’re back to your original, unactivated shell.
Your best option would be to do it in a function
activate () {
. ../.env/bin/activate
}
or an alias
alias activate=". ../.env/bin/activate"
Hope this helps.