Why can't format old floppy disks?

[Circlotron]

Got a pile of old 3-1/2 inch diskettes. May of them date to the mid 90s, some even earlier. Nothing important on them. A few I can actually read the table of contents, even fewer I can actually read some of the files. Presumably the magnetic domains on the disk have gotten weak over the years. I can understand that. But why wouldn't they be able to be formatted? I thought that would make them good as new. Totally blank of course, but quite usable if I wanted to.

[TERRA Operative]

Probably physical degradation due to outgassing/oxidation/mold/etc.

Also, make sure your read heads in your floppy drive are perfectly clean (isopropyl alcohol is useful here) and the mechanical parts arw well lubricated.

[DavidKo]

It was quite common even in old days. To find disk without bad sectors is quite tricky.
Based on this site DiskDupe can help you.

[mikeselectricstuff]

ISTR 720K and 1.4M disks had different magnetic characteristics, so maybe some compatibility issues using the wrong type?

[xmris]

@OP this may help you, I also have a lot of floppies most of them purchased early 90s and I managed to reformat a lot of them. Btw as mentioned above, mould is an issue, esp ON the floppies, check them for signs of it, then clean -gently- with some cotton pad with IPA.