I am not a teacher.
"It looks like it's been there forever." means quite literally that something has the appearance of having been in place for all time. This is usually used to describe something that looks old, antique, or worn.
It could be either a good or a bad thing, it would depend on who said it and what they were describing.
For "Oh, who wouldn't like that tub? It looks like it's been there forever.", this was likely said by someone who appreciates old things, and views them as having value. An old antique tub which looks so old it must have been there for all time.
In another statement "Why would anyone like that tub, it looks like it has been there forever". This would be said by someone who does NOT like the old tub, and would rather have a new tub.