Why do I keep losing disk space even as I delete files?

There is " Purgeable Space " which is under the Control of the Operating System.

And then there is " Empty Space "

When the Operating System decides Empty Space is needed, it will release portions of the Purgeable Space back to the Empty Space category to be reused as needed.

AFAIK - there is not way the User can hasten the transfer of the Purgeable Space back to Empty Space except " Give it a day or so to settle down, as the Finder takes as much as 24 hours while it recalculates and eventually releases the space." to quote the good advise from previous Contributor

To add a little more on clearing System Cache files which do occupy space on the drive

Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

Safe Mode will also eliminate Third Party Software, extensions and drivers from loading. It will only load the Minimum amount of Core Apple Processes to allow the the computer to function at a reduced Level of Performance

The System cache files will be recreated as the System Requires

I understand that you have been deleting files in an effort to increase space, but it seems the opposite is happening. Give it a day or so to settle down, as the Finder takes as much as 24 hours while it recalculates and eventually releases the space.