It's considered sport first, because transportation by bike is
usually impractical in the US. (Outside of maybe a half-dozen cities and some one-off "I work a mile from home" anecdotes, it's almost entirely impractical)
The average commute (one-way) is about 25km. There will be no bike lanes. The drivers on the road will generally have no clue how they should deal with you, if they even notice you. The sidewalks (where US bikers sometimes find refuge, as no-one walks in America either) are sporadic, at best. Your destination probably won't have so much as a simple bike rack, let alone a shower.
Because of this, when adults in America think of biking, they (generally) think of putting their bikes on the car and driving to a place that was designed to have decent/sane space for biking. (usually parks)
Children can effectively bike around subdivisions, mostly because they have nowhere to go aside from their friends houses and the corner store.
So biking is seen by adults as either being sport, or --frankly-- a whimsical sort of transport. Because to bike in most the US you have to be, in some sense, not serious about trying to get anywhere.