Also, the two chambers are not equal. The Senate is very clearly the upper chamber.
In particular, for any bill requiring spending, the Senate can only amend a bill that started in the House. Impeachment must start in the house and is then tried in the Senate. The Senate can't nominate someone to office, they can only confirm a nominee presented by the executive.
Words have histories of course, and probably at one point this was a spatial metaphor - maybe some bicameral legislature literally did have one of their higher status body on a different floor than their lower status body. But the words upper and lower when applied to the legislative branch have evolved since then.