Originally Posted by Trakhak

Blame racers for what?

Bike frames were initially brazed/welded without lugs, using tubing that had to be carefully mitered to fit together at the tube ends. The use of lugs was then introduced as a cost-cutting measure to eliminate the need for the time-consuming mitering step (or at least to reduce the need for precise mitering).

Building bikes with a horizontal top tube and parallel head and seat tubes helped reduce the range of lugs a bike manufacturer would have to keep in stock. In fact, for the vast majority of bikes (i.e., for the utility bikes used for transportation rather than for sport riding or racing), one set of lugs would suffice for all sizes, with longer or shorter head tubes and seat tubes used for taller or shorter riders and tubes of unvarying length used for the top and down tubes. Further simplifying production, utility bikes were usually manufactured in at most two or three frame sizes.

Drop bar racing bikes were of necessity manufactured with smaller increments between frame sizes and with varying lug angles and tubing lengths, but the practice of maintaining a horizontal top tube eliminated at least that variable. Standardization!

Oh, and BMX bikes, which came along much, much later, were cheap to manufacture without lugs because, although precise mitering of the tubes was required, the bikes were usually built in only one size, which meant that manufacturers needed only one complete set of mitered tubes in inventory.