why only 5

Why? What's the logic behind that? I'm not trying to a big but 5 isn't a set number it just works out that way...

There are only 5 houses that are comparable to one another for the most part in the top tier. You can compare aphi to pi phi, but try to mix a house like AXO in there and it just wouldn't work. They dont recruit the same girls

I think the tiers do come in to play by who is recruited. It's true that a girl attending pref parties at some level of houses most likely isn't attending prefs at another level of houses. Girls who are attending pref parties usually follow along these lines (ie the houses are attracted to the same girls): top tier houses are attracted to girls who end up pledging (from actual recruitment data)
Example scenario 1: kjzPi Phi, A Phi, DZ, Theta, AXO
Scenario 2: DG, Kappa, Chi O, Theta
And so on...through the tiers

Yes, some are upper middle tier houses. As girls can choose up to three houses for pref rounds.... Most have a combo of the above houses for their pref round of top tier. Girls successfully rushing mid tier houses will have a mix of upper & mid tier houses. Then there are those that are rushing low to mid tier houses and they will have a mix of those for pref. It's extremely rare that a girl has a top tier house and the rest low tier for pref unless there is an internal reason the top tier house is inviting that girl back. It just goes to show that the houses are consistent in who they rush so be yourself! You can't fake who you are and the girl's in the houses do know who will fit nicely in their house.
BTW tiers become known by how many girls in recruitment want to go back to a specific house after each round. They are considered the popular houses by the girls rushing.