Why can't videos and pictures be removed from the smartphone?

Over time, of course, a lot of files have accumulated on my smartphone (HTC one). I wanted to tidy up and pulled these nearly 7000 files on the laptop. I have selected in the settings that they are deleted from the smartphone. Result: I have 7000 more or less useless files on the machine, and they are still on the smartphone. If I take a single picture over then it will, if I previously set it, deleted on the smartphone. Why does not this work if I transfer all files? Who knows advice?

A device can't delete data, but only release the old files for overwriting and then overwrite them with other data.

You would probably have to use a special software that overwrites the shared space.

Maybe the files no longer exist on the smartphone, but are displayed in the gallery because they are stored in a cloud (for example, Microsoft Onedrive, Samsung Cloud, etc.)?

But it is actually the case that a file on the smartphone is no longer present / findable, if I pulled it on the computer.

I think that they are on the smartphone. The stores are also pretty full.