Because it would be obnoxious to detach it and attach it everyday. I just want to have it outside together with the bike (attached to the bike) but only detachable with a key, and of a material that won't rot after rain.
Sounds like you're after something different than a traditional trailer.
Maybe a different type of bike altogether:
or a trailer like this:
I'm sure you could lock it to the bike somehow.
Or an extra cycle:
