My understanding was that IDLE comes built in with Python, which is installed with Ubuntu. I can't find IDLE in Applications. I can't find it to install in Ubuntu Software. Where is it? Not interested in workarounds. Want to know why it isn't there. Ubuntu 18.04
Did you try installing it from the command line directly, as this other almost identical question/answer pair instruct you on how to do?
You can install IDLE for Python 3 with
sudo apt install idle
... or with ...
sudo apt install idle3
There is no Python2 version of IDLE in Ubuntu 18.04, however.