They are like toads, they perfer to hide most of time. They really should be called "chubby toads" .. heh.

Hiding doesn't mean they are stressed, it's just more comfortable for them to be buried in substrate or in a hidey log.

It also doesn't help that they are a nocturnal species.

On that note though, if you do have kids, or the tank is a in a high noise/traffic room they might be extra hidey. Might want to move them to a less used room and it might improve how often they are out.