Whereas there are MP3 player-to-tape player adapters, is there an MP3-to-CD player adapter? If not, why the heck not?

Most adapters use either an auxiliary input or transmit an FM signal to the car player. They are quite common and easy to find.

It’s a problem of interface.

1. Tapes do not spin at high speeds
2. Tapes are relatively thick

You would need to have a 1.2mm thick “CD adapter” that was able to keep its cables attached while spinning at high speeds (200–500 rpm).

Additionally, the “track” on a Tape is 0.6 mm wide, moving at 4.76cm/sec, on a CD the “track” is 500 nm wide and it moves at 1.2m/sec

For comparison, that means the track on a CD is 1200x smaller than a tape track, and it moves 25x faster.

It may be possible to build a CD adapter that has a central hub that spins, meaning the outside doesn’t spin, but then you’d need to identify where the laser head is, and present it with the correct optical data stream.

Plus it’d need to be no thicker than 1.2mm. Most printed circuit boards are at a minimum 0.8mm thick, and components ontop of that would be at least another 1–2mm.

I’ve always assumed that you could plug another audio source into a tape deck because a cassette tape is stationary, you just have the head of the tape player touching an electronic transmitter. But since a CD spins, that seems it would pose a far greater challenge, as I see @grumpyfish has explained in great detail above. I agree with @DarkScribe, get an FM adapter, I have one and it works great.

You know, you might be able to do this with a very clever cuttlefish.

Well, the MP3 to CD adapter head would not really have to spin. It would just have to give the right reflective outputs to the laser in the CD player. The player normally shoots a laser and picks up reflections from the pits in a CD. Flashing pits are translated to digital data, which are translated to sound. You would have to duplicate that to fool the player. ... and do it with an adapter 1.2mm thick. Very difficult.

MP3 to tape adapters have to interface with the (stationary) magnetic tape pickup head in the tape player. It’s pretty easy to run the electrical output from an MP3 player into a magnetic coil. The adapter just needs to place the coil on the tape pickup head.

@RocketGuy Add to that the need to locate the laser head (no standard location), so you’d probably need to have the entire surface pulse… ergo, clever cuttlefish with an audio-in jack.