Possibly because lodgers are generally not permanent residents and may well move on. It would mean that the owners income could drop quite a bit...or the lodger is only paying a few pounds so it is not worth considering.
I would think that they would expect adult children to move out of the parental home as soon as they are financially settled or in a relationship etc...and lets be honest few parents would throw their children out if they stopped paying their rent through, for example, job loss. So not a permanent income.