Question:

Why do you suppose small-ring cycloalkenes such as

cyclohexenedon't exist as cis-trans isomers?

Cyclohexene doesn't exhibit this type of Isomerism. Cyclohexene is a tiny ring to exhibit the trans isomer. The Structural formula and component bonding characteristic within it limits it to show this phenomenon. Trying to force it to be trans would place a massive amount of ring strain on the molecule.

This characterises major rings within Alkene group up to cyclooctene.