Pipe and tube

[Toolmaker51]

Probably a tremendous over simplification, but here it is. As pipe and tube were utilized differently, a strict uniformity wouldn't have appeared sensible. Pipe schedules relate to pressure, so regardless size they'll contain the same force. Wall thickness and internal surface area and inextricably connected. The connections are the weak link whether slip or threaded. Pipe threads are not intended to create a mechanical connection in the way nuts and bolts do. Piping requires support to protect the joints, the taper of pipe threads is less about leak proof than trying to align cylindrical threads; attaining a seal benefits from the taper secondarily. Having assembled fire sprinklers, full lengths, overhead, countless fittings, etc makes one appreciate a system intimately. Scheduled pipe does reduce field efforts, at time of engineering and permit inspections, and when replacement occurs, instead of potential miscalculations.
Tubing structure designs consider torsion, compression, etc; mechanical forces controlled by wall thickness and external surface area whereas connected to other forms, such as plate where machining and welding form the joints. Most structures require heavier material foundation/ bases that could reduce proportionally with height. Wall thickness and/ or taper accomplish that. It's easy to purchase cylindrically telescopic sizes of round, square, and rectangular tube. Very long 'tubes' are now formed in specific faceted tapers, as used in large wind generators, high tension electrical towers. Round tapers have been around longer, such as flag poles. Size limits in length are imposed by what can be transported over the road, diameters by bridges en-route to construction sites.
Another basis is older. The societies bent on colonization imposed measurement on subject colonial territories, which influenced development of tools and materials from there on. The more isolationist groups concentrated on logical standardization, whether adopted as simply convenient or wholly anew. The empires drove both, as trade would dictate, being more or less in control of production.
WWII began the first real documentation of incompatible components, especially mechanical fasteners which occurred first. The differences were retained; with perfectly accurate representation of dimensions so a producer could complete parts intended for a distant user. There have been successive meetings, notably in the 60's and 70's. Can't imagine the volume of paper and pencils consumed before pocket calculators...
Marv's observation on drill sizes is correct, with an additional condition in colonial business. Not to say much logic or foresight is evident. That was to meet demand of increasingly diverse products and processes; in physical size, complexity, and fits desired for functional purposes; multiplying in both directions.