Yes — the opposite of a teleconverter is generally called a focal reducer, and one well-known example is the Metabones Speed Booster.

In principle, it concentrates the image circle from a full-frame lens onto a smaller sensor, which makes the effective field of view wider and can increase the effective brightness.

The catch for DSLRs is physical space. A focal reducer needs optical elements placed in the path between lens and sensor, and on DSLR mounts that space is limited by the mirror box and shutter. That makes focal reducers much harder to implement than teleconverters, and in many DSLR systems they may be impractical or require complex optics. Adding extra glass can also reduce image quality.

So the idea is real and not forbidden by physics, but DSLR design often makes it difficult. They are more commonly seen where mount geometry leaves enough room for the adapter optics.