:wasalamex
I've heard lot's of people question that, and i've thought about it myself. But i was listening to the hereafter series by Imaam Anwar al-Awlaki and he was saying that we just can't imagine the hereafter, because we can only compare things to this world, so we have vertical, horizontal, diagonal - but in paradise, there could be even more dimensions than that.
I think it was Ibn Abbaas [the cousin of the Messenger of Allaah (peace be upon him) and the teacher of Qur'an] he said that things are only similar by name, so when it's mentioned that tree's will have golden trunks, it won't be like the gold of this world. But it has to be described in a way so we know that it's worth something, so we want to strive for it.
We can't understand the structure of the houses there, we don't know what dimensions they'll be in. So maybe we can't understand this because we can only measure according to the limits of this world?
I even think to myself - why is this description repeated so much? Allaah knows best, but if something is repeated continuously - it represents that thing. So whenever 'beneath which rivers flow' is mentioned, i already know that it's paradise. Even though its not clearly stated. The same way whenever the word 'khaalidoon'' [(they will) abide therein] is mentioned, i all of a sudden remember hellfire. Why? Because this is repeatedly used by Allaah to describe that the punishment of hellfire is for a really long time.
You see how Allaah doesn't really explain paradise or hellfire in too much detail, rather it is used alot of times with a similar description. That way we associate this with paradise due to it being repeatedly continuously. The same way a company doesn't keep changing its logo after every month, rather it keeps the same logo so people recognise it still.
The arab's used to live in the desert and they desired to have gardens, rivers etc. Because clans would fight each other for decades just for a well of water. They wanted fruits, and shade etc. So maybe by making this association of gardens and rivers made them think positively about paradise because they longed for something like that, so it was worth striving for. Allaah Almighty knows best.