Nice Question :))
Short answer: Density - It just takes way too much cards to store anything useful.
(And no, there is no way back in the good old time of optimized data structures)
When you think about it, punch cards are the safest way to backup data for long term storage.
As usual that depends on your definition on 'safest way'.
They are not influenced by magnetic fields and their data (the holes) don't fade over time. They are also less sensitive to heat than most backup media I can think of.
Like any other media they do need a careful storage. But you're right, they can endure quite some abuse and neglect.
Their biggest advantage should be that they are future-proof. Try reading an old computer backup tape or diskette (pick any size). Even if the data signal hasn't faded you probably cannot retrieve the data without some major effort.
Not true for classic nine track Tapes. they can not only be read by very primitive means (ferro fluid and a set of glasses), but are also (usually) plain text data. Of course, every data on every media may be application specific, and thus hard to decode for future data archeologists.
Punch cards are easy enough to read and don't have proprietary encoding issues.
Sure they have. Just because one physical storage is the same for all usages (size of cards and holes) doesn't mean all encodings are the same. While regular data may be encoded using zero to three holes per column to encode one symbols, there have been not only extended encodings with up to six holes per column for binary data. But even within just one supplier like IBM there were many ways to use the card. Including horizontal storage, where data words where stored in rows, not columns. And then there were other manufacturers. All using the same paper and hole dimensions, but completely incompatible encoding.
If you wanted to read your grandfather's punch cards (or: your grandson wanted to read yours) it should be easy enough to create a card reader. It may not be ideal for graphics, but data - e.g. a family tree or government records - would be much safer if stored on punch cards in a fire-proof box.
Or even use metal cards, stretching the durability even further.
How come punch cards are not used (or not very popular) for long-term backups?
The main reason is density - or it's reciprocal value size - punch dards carry only a minuscle amount of information compared to their size.
A punch card is 7.375 by 3.25 by 0.007 inches (187.325 mm x 82.55 mm x 0,18 mm). That's a volume of ~2.8 cm³. With 80 bytes stored per card (*1), this comes down to 0.035 cm³ per character.
Thanks to the metric system everything from here only involve shifting the magnitue (*2).
- One KiB is like 36 cm³ - like a small wallet
- One MiB is 36,700 cm³ - already the size of a suit case (~10 US gallons)
- One GiB is 37.58 m³ - that's already almost the size of a garage.
- One TiB is 38,500 m³ - That's the size of a small school sports hall.
All of this without taking boxes and other measues to handle them.
Now tell me again, what's the size of your actual hard drive(s) 1 TB? 4 TB ?
*1 - We assume a simple standard code, no optimization that may give future readers a hard time.
*2 - I still used a pocket calculator, despite the fact that its a rough estimate anyway.
Then there is paper tape. Their density is about 10+ times of punch cards. Sure, a bit more fragile, but not so much. Still, even with 10 times less space needed, it can't be really used for today's surge of data.
Regarding the title ("Why no punch card readers in use?" ): They are ... at least I got one sitting right behind me, fully functional and in use.