Why do we clock Buck Converters?
There are lots of hysteric or modified hysteric buck converters available.  For example take a look at TI's DCAP constant-on time converters:

TPS53355

Or a more conventional true hysteric buck converter:

LM3485

Hysteric buck converters actually require some minimum ESR in the output caps for stability, so they tend not to work well with ceramic output capacitors. (Without some modification.)

Also in a true hysteric converter (not as much with the COT approach) the switching frequency isn't constant.  This can be a problem at light load when the switching frequency may get down into the audio band causing audible whine or noise.  It may also cause interference with other circuitry at certain frequencies.  

Because of that it's also difficult to filter conducted noise.