Storing your audio on magnetic tape, rather than on a flash EEPROM type device is looked at by some as being "old school". While this may be the case, there are valid reasons that we choose to stick to recording our audio in this format.
While the legal system has made great strides in recent years in adapting to new technologies, there are some valid reasons that make taped recordings more likely to be accepted as valid evidence by a court of law.
One of the major reasons is the detectability by experts of audio tampering on a cassette tape, that is simply not possible with a solid state memory device. Because magnetic recording is an analog medium, traces of tampering are more difficult to hide than a computer memory device where the contents can be completely rewritten with no trace of the rewrites.
There is substantial precendent for admitting audio tape evidence, so courts are comfortable with it. Digital recordings face a higher bar of admission. A "Chain of Custody" needs to be established, like for blood evidence. A proper and trusted chain of custody is easy for a law enforcement agency to provide, but is almost impoosible for regular people. That means the court has less assurance that the recording hasn't been tampered with, and absent of the ability for a forensic expert to validate the recording, it's admission becomes less likely.
While it is likely that 20 years from now, it may be difficult to find audio cassette players to play your cassettes, and while 20 years from now, the physical strengh of the tape itself will have been reduced, making breakage more likely, it's still true that recording permanence is a reason to use audio tape.
Digital recordings also suffer aging and obsolescence effects. Try finding a zip drive, for example. It is likely that 20 years from now, the digital media that is now available, will be very difficult to find devices to read.
To say something is "also bad", is insufficient to use it's competitor. The reason to use magnetic tape over digital has more to do with short term permanence. Events like electrostatic discharge (the shock you feel when you touch a doorknob) can render digital media unreadable. Even if most of the bits themselves are left intact, damage to the address or data lines needed to access those bits renders it just as unreadable as total destruction of the memory itself would.
There's also the issue of data corruption. When an audio tape experiences some loss of data, the result is often inaudible. It may also take the form of a brief silence in the played audio, or a pop or crack in the played audio. When a digital device experiences some loss of data, it is certainly possible to detect the bad data and ignore it. However, often, the device ends up generating an error and halting the audio playback. This is recoverable by trained engineers with expensive equipment, but is generally a virtual data loss for the rest of us.