Why is a bicycle a frame structure?
Take a look at the longest lasting structures possibly known to man...the pyramids in Egypt. What shape are they? A 4 sided triangle. What shape is a bicycle frame? A triangle. To 'lowlevel' who states a bicycle "doesn’t go fast enough for aerodynamics to be of huge concern..." Bull 💩 Sh*t! Quote, "In a solo event or triathlon, lowering total aerodynamic drag by 10% (from 7lbs of drag to 6.3lbs), without changing power output, will cut 21 minutes of time (7%) from a rider who averages 20mph over 100 miles. Time will drop from 5 hours to 4 hours and 39 minutes and average speed will go up to 21.4mph." https://fitwerx.com/bicycle-aerodynamics/ More? Here's "Six ways to improve aerodynamics on the bike" - https://roadcyclinguk.com/gear/six-ways-improve-aerodynamics-bike.html It's a frame structure because it has to hold WEIGHT, just like a bridge. And the triangle shape is the lightest & strongest shape designed by man...so far.
Among other things, to reduce wind resistance from the sides. If an object as tall, light and thin as a bicycle presented solid faces to the sides, it would be liable to be blown over by a gust of wind. This is why disc wheels are only used for stadium racing, which is sheltered from winds. The wind cannot pass through them as it does through spoked wheels.
Because it has a frame.
This is really more of an engineering question than it is a bicycle question. You may want to use the edit icon to move your question to the Science & Mathematics>>Engineering section.
Is this on the test?
Because it doesn’t need to be anything else. A frame structure is always the lightest. You look at bridges, towers, cranes, etc... and they always have frame structures. The only reason to enclose the frame, or use a structure with a load bearing skin is for protection from the elements (buildings, cars) or for aerodynamic purposes (aircraft, spacecraft which are launched through the atmosphere). A bicycle is designed to be light weight, doesn’t go fast enough for aerodynamics to be of huge concern, and could not protect the rider without completely destroying its weight and aerodynamic advantages.