Why is the itl file not in my itunes backup

At a guess you have a split library where your library files are on the system drive, and the media on an external. You will only have a backup if you're copying the entire library, database and media files, to a secondary location. See Backup your iTunes for Windows library with SyncToy - Apple Community and Make a split library portable - Apple Community for more.

tt2

See Empty/corrupt iTunes library after upgrade/crash - Apple Community. Perhaps there is a useful file in the Previous iTunes Libraries folder, though these might be rather out of date, or a hidden .tmp file that contains a recent version of your database, or you may be able to recover playlists from a device.

tt2

Apple's backup advice suggests consolidating your library into an iTunes Media folder that is assumed to be inside your main iTunes library folder, and then copying the iTunes folder to another drive for backup. If you copy the iTunes Media folder then you're not capturing the library files which are either in the iTunes folder that contains iTunes Media, or perhaps some other location depending on how you've moved things around in the past.

tt2

It is covered in the article, but make a copy, rename it as iTunes Library.itl, and then launch iTunes to see if it is acceptable. Work with the most recent .tmp file that is comparable in size to those in the Previous iTunes Libraries folder. Any .tmp file you see could potentially be the .itl, .itdb, or .xml files in the process of being updated.

tt2